Water hose - BPA Free (RV hose)
Get a white or blue "Pure Water" or "Taste Pure" or other "RV type" water hose that is free of harmful BPA chemicals. Store it isolated, well from any contaminate potential like the stinky slinky, rugs, or other things often found in storage compartments.
Examples

Water hose gaskets
You know them as either impossible to get out of the end of the hose or nozzle, or so loose they fall out every time you disassemble. Pick up a bag full or a 12-pack. You'll probably thank me at some point.
Examples

Water line pressure regulator
Campgrounds, and even home water lines, may run at high enough pressure to damage the water lines inside your rig. We've had internal water leaks due to inadequate assembly on two RVs in the last few years, possibly brought out by high campground water pressure. Normally such leaks would be caught by the manufacturer's functioinal testing, assuming such tests are conducted, but we had a leak under the sinks of a Micro Minnie Winnie and a Cougar, both known as quality product lines. Do yourself and your rig a favor, and add an inline water pressure regulator. Make sure you get one with a rock screen / filter on the front end. We stayed at a brand new resort in late 2020, and every time I disconnected the water I had to clean out the little rock filter/screen. It was full of tiny gravel from the new lines that could have wedged themselves into the regulator. And yes, I installed that upstream from the filter (that would do the same thing) to keep potentially high water pressure from "expanding" the in-line water filter.

It's a good idea to have the flexible extension shown alongside the water filter (below) in case the regulator's rock screen doesn't fit into the water faucet well enough to seal properly. Sometimes that happens when the site has an anti-siphon valve permanently attached. Just put the extension in between the faucet and the regulator. Example 1 Inline Regulator
Example 2 Inline Regulator
Example 3 Inline Regulator + Gauge
Example 4 Variable Inline Regulator + Gauge

Water line anti-siphon / backflow preventer
Many parks already have these attached to the spigots to protect the infrastructure from accidental poisoning. Be a good steward and buy your own to make sure you never backflow anything into the water system. If you have a black tank flush fitting, buy another anti-siphon valve and dedicate it solely to the black tank role. Make absolutely sure to mark/label it for this application only, or leave it attached to the flush port. And remove the setscrew, it's only necessary for permanent mounts and will mess up the threads on your hose.

Water filter
These should be used for anything you put into the tanks or lines, to eliminate smells, tastes, particulate and protect the rig and most of your water fittings.
Example with hose protector

Water pressure gauge
Understanding the incoming water pressure can help you identify and/or avoid many plumbing challenges. It's nice (for me, as an engineer) to know both the pre and post regulator pressures, but common sense says the most valuable place to measure is post-regulator, to know what goes into your rig. There's no need for the bluetooth or wifi models unless you're willing to admit to being geekier than me..
Example

Water line adapters: 90 degree
This might be as simple as a right angle elbow to let your waterhose drop straight down along the camper wall and reduce the stress on the connector. You never want to hang the water filter or the weight of the (full) water hose out from the rig, that's inviting cracks and leaks. My current Cougar has a water connection door with freshwater in, black tank flush in, and a hot/cold faucet line out. I use an assortment of fittings to let the lines pass through the door's openings, while letting the hoses hang nicely down to the manifold. So if you buy one, get a few more to make the line run where you want it. Some even have built-in valves.
Water Hose Elbows

Water "Bandit"
You will eventually come across water hoses lacking a fitting or ending in a damaged fitting. This doohicky will let you seal your hose to it with little or no leakage. If you ever intend to use it with your tank flush hose, buy a second, label it, and keep it with your stinky supplies.

Hose Protector / Short water hose with kink eliminator spring (BPA FREE!)

Water hose splitter/manifold, 2, 3 or 4 way (gauge, fresh, flush, rinse)

Many Examples
Even one with WiFi!

Clean Water / "Used" Water Isolation
Most rigs have a pass-through for storage. All rigs require storage for clean water items. All rigs require storage for not-so-clean water items. Keep the stinky slinky hoses in the bumper, dedicated outside storage, or in a sealable tote in the back of your truck or on the bumper. Smaller (dedicated) used water connectors stay in a bag on the "dirty" side of our pass-through. Clean water fittings, hoses and filters stay in bags or wound up on the other end of the pass-through. And they never get mixed even for a second's convenience.

Cleaning supplies in a tub
Two flavors here. One is for the storage of interior cleaning supplies stored in a tote with a handle. Solid bottom to keep leaky chemicals from staining the carpet or flooring. The other flavor is for the more personal cleaning supplies that often reside in the shower. A similar handled tote will work, but you can store the tote in the shower (if there's room) by drilling a few holes in the bottom corners to let it drain.

Small storage tubs for organization in closets/overheads
IKEA's SAMLA series is clear, lightweight plastic in an a range of sizes, all stackable to some degree.
Clear containers are a "clear" benefit to knowing what's inside.

Spare / extra blankets, towels, pillows, clothing
RVs aren't nearly as climate controlled as a home so it makes sense to have a way to provide heat/warmth is specific locations. Likewise during the summer, it might be nice to spread out the bodies, so spare pillows and sheets add to the flexibility coefficient.
The more years we camp, the more we dedicate to the operation. The items live in interior storage tubs after cleaning. The rig gets parked in the driveway for a night or two upon return, during which everything is cleaned and replaced into the right spots ready for the next adventure.

Electric Plug adapters
If your rig is 50A, you'll need a 50A to 30A adapter. If you have a 30A rig, you'll need a 30A to 50A adapter, and a 30A to 110 outlet adapter for barebones charging at home. (110V=>30A=>50A and 50A=>30A=>110V)

Surge Protector
I'm torn between describing this as mandatory or a potential upgrade. Some experts say to always use one. We've RV'd for decades from Washington to Florida, from California to Kentucky and never needed one. But it only takes one event to ruin your day. External corded models from $100-300 are good insurance and in some places you'll want to lock them up. I'm planning to add an internal unit to avoid the hassle and storage requirements. Our coach had one built in like many higher end rigs.

Battery Charger or a solar solution
Your battery WILL drain at some point, if you can't plug the rig in you will need to be able to charge the battery outside the rig, so a 10/6/2A or even a trickle charger will make that possible.

100 Watt (min) solar panel + charge controller
That's just to keep the basics online. If you plan to really use the system off grid you'll need several times that. Example, a Keurig draws 1500W to startup, most solar systems can't support that directly so you'll need to be able to charge the battery for 1500W/XWatts where X is the capacity of your solar system. A 300W system will need to charge the battery for 5 times as long as the Keurig runs, not counting any other losses. If the Keurig runs for 2 minutes, you'll need full sunlight and no other load for 10 minutes between mugs. It's actually a little more complicated, but solar is a slow, continuous charge while the loads tend to be intermittent and larger. More on this later. And, it only charges in daylight...

Inverter / Transfer Switch / Big Battery
It's changing, but right now many/most RVs aren't set up for enjoying 110V appliances while boondocking or overnighting without an incoming AC feed. An inverter will attach to the battery system and provide 110V AC for a few outlets if your rig is pre-wired for it. Ours has a loop in the basement that allows insertion of a Transfer Switch between the breaker box and 4 or 5 outlet. I set up an automatic transfer switch to default to shore power but cut over to the inverter when it gets turned on. The conversion from the 12V battery supply to 110V AC requires a lot of battery capability. Example, our 2000W inverter could draw a peak of more than 110 amps from the battery to power something like a Keurig coffee maker. That means very large wire and fuses between the battery and inverter. It also means a very large battery (or several) if the load occurs very often.

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Get some gloves
The 6mil thick black nitrile gloces work surprisingly well. Yes, you'll sweat, but personal cleanup is much easier.
Example

Stinky Slinky / Sewer hose
No matter how tempting it might be to save a few bucks, do not, and I mean DO NOT buy the hose without permanently attached fittings. I like the Valterra Dominator hoses for the ability to collapse and stay collapsed. It makes them easy to route and they generally stay in place. The 10 foot sections collapse to less than 3 1/2 feet. Almost all hoses use the standard bayonet fittings, although some may be tighter than others. You want a snug connection, but they should separate without the use of cheater bars. And don't skimp on the quality, remember the scene in the movie "RV"?

90 degree clear elbow
Let the fun begin

Gasket
This goes between the elbow and the sewer opening. Some sewers have threaded PVC pipe that fits the elbow perfectly, the gasket is for those that don't. Be prepared.

RV Toilet paper
Some experts say special toilet paper isn't needed if you use plenty of water. Other say to always use RV toilet paper and plenty of water. The RVTP is thinner than the luxurious stuff advertised on TV but it is designed to dissolve more completely. We have survived using it since 1997 with no clogs to show off. Also available at virtually every Wal-Mart in the country.

Black tank deodorizer
You want this or something like it before your first trip. drop it in the potty and add a couple of gallons of water. Simple as that. This one's even Orange "Flavored". The plastic wrappers can build up, so make sure you can flush the tank well. There are also liquid deodorizers as an alternative. Only disadvantage of the liquid is that it's not quite as convenient.

Note: This is no substitute for using plenty of water to flush the black tank. Copious water is your friend.

5 or 10 inch extension in black or clear for ease in hose attachment and/or flush verification. It makes attaching the hose much more pleasant if the dump port is way up under the rig.

Examples: Valterra 10 inch, Black or Camco 5 inch, Clear

Magnetic bumper covers to retain sewer hose
For two years I have recommended these, but when I picked up the trailer the other day one was missing. I don't know if it fell off while driving, but a side marker light on the same side was also missing. Not sure if it was due to wind or pranksters.

Sewer hose 'Y' adapter
Designed for multiple gray tanks on larger rigs. If this applies to you, and you intend to stay in one place for several dumps or just want to be able to leave the hoses set up during your stay, you will want to add this Y fitting and additional hoses to your collection. There's nothing preventing you from moving the hoses from front to back when dumping, but consider this a serious convenience enhancement. We used it for several months waiting on a home build to complete, but have only used it once since.

Leveling Blocks

Wheel Chocks
Rubber wheel chocks are ideal. Plastic chocks will work on some surfaces but you'll eventually want to upgrade to rubber for concrete. I now travel with 4 rubber for concrete pads and driveways, and 4 plastic for gravel and dirt.

Locks for the tongue, hitch
Preferably with matching keys

Jack for changing the trailer tires.
It might be a simple as using the leveling blocks on the adjacent tire when tandem, it might be a real scissor jack, or ideally the built-in electric jacks.

First Aid Kit

Flashlights: magnetic, rechargeable (maybe USB)
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Pliers
needlenose, channel-locks, diagonal cutters, crescent wrench

Combination Wrench large enough for hitch repairs

Screwdrivers
P1, P2, large and small blade, Torx bit set

Small and large pry bar
Or body and trim tool - Plastic

Box Cutter
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Multimeter for AC and DC voltages, and continuity/ohmmeter
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Combo toolset with 1/4 in and 3/8 in ratchets, 1/4 inch drill adapters for the sockets
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cordless drill/driver, with a jack adapter for the drill.
This is an awesome setup if you have manual jacks, but the cordless drill/driver is a necessity anyway.

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12V air compressor and a compatible plugs

Wire stripper and wire crimper

Heat Shrink tubing + solder connectors

Spare fuses and lightbulbs

Silicone caulk

Boeshield T-9 Rust and Corrosion Protection/Inhibitor and Waterproof Lubrication
It is expensive but well worth the added cost. Get the two pack, you'll want it in your garage, also.