A set of replacement fuses. Preferably several in each amperage. Blowing a fuse on one of the 12v
circuits when you've got no spares can bring the fun to a halt depending on what the fuse was for. On my very first camping trip in my brand-new Airstream, I blew the same fuse twice before I figured out what I was doing wrong.
Equipping any RV is an ongoing process, not a do-it-once-and-you're-done process. Start out with sinjin's list, add fuses.
Add lawn chairs so you can sit outside and enjoy the campfire.
Add clothing, unless you're a naturist. Hospital scrubs make good all-purpose clothing. You can use scrubs as pajamas, lounge wear around the campfire, coveralls for messy jobs. They're lightweight, comfortable, dry quickly when wet, and who cares if they get wrinkled?
Add a few days' worth of food unless you're fasting.
Add a generator if you're boondocking for more than a few days.
Add insect repellent, DEET for your skin, permethrin for your clothing and bedding. Being bitten by tiny vermin takes the fun out of anything.
Add a camera so you can post pictures to the Forums. That's the item I forget most often.
Add a notepad and something to write with, so you can take notes and add to your, "I think I should have brought _____" list. And trust me, you WILL have such a list.
You'll also eventually have a "Why on Earth did I bring _____" list of things to leave at home next time. Anything you bring but never useŚ except safety equipment that you hope you'll never use but need to have anywayŚ can come out of your Airstream. But don't get rid of it altogether, because it may go back
on your "I think I should have brought _____" list someday.