If you are expecting a higher resale for the RV because you've included an already outfitted unit, you might be waiting for a buyer for a long time, if ever. There may be exceptions, but normally, RV's are sold empty, and at best you get a starter kit, which is a sewer hose, a pressure regulator, and a toilet paper sample.
However, buying-selling from a private individual, well ... anything is permissible. But don't expect more money for the sale because it is outfitted with items that worked for you, but may be total junk to the new buyer. The new buyer may say, take the 'junk' and knock off $2000.00. Here again though, it's your sale, you can do whatever you want, just don't be upset at people because they are not accepting of the stuff you found valuable.
If you have "stuff" it makes better sense to part with it, Good Will, give it away to a fellow camper, yard sale it, or trash it. If you have some items you really want to get rid of, tell the buyer those things go with the camper, but you won't get any more $$$ out of it anyway, as the alternative was the dumpster.
We've accumulated a lot of "stuff" over the years. Tent camping, to Pop-up, 3 travel trailers and a 5er. Each step to a new camping style meant getting rid of stuff we no longer wanted or needed and getting new for what we did.
We gave everything away. When our last travel trailer had an outside kitchen sink and propane stove, we finally got rid of all our Coleman camp stove stuff. Actually, because of an on-line swap group my wife belong to, we ended up giving the two Coleman camp stoves, all the hoses, a 20 pound propane tank, propane T's, regulators, a gas lantern, the whole enchilada to a local Boy Scout troop.
I gave away all our Dish Sattlite system stuff, including the antenas, tripods, receiver, wires, everything. Just about 2 months ago I gave away a 1400 / 14000 pound Equal-i-zer weight distribution hitch to a colleague at work who just bought a new travel trailer. I have a 5er, no need for it any more.
Just a couple weeks ago, I gave away the last few items I had at a campground. I walked around and found a trailer that could use those wind flappers for their awning. It's stuff like that, we've giving away to fellow campers as we've no longer needed it.
We figure, we have received our money's worth out of the item. We will never use it again. "It" served us well. We expect nothing back now. Give it to someone else who can benefit using it, and save a few dollars.
That's how we've always done it.
When the time comes to hang it all up, we'll probably just give away everything we've got. Leopards don't change their spots, no matter how old they get.