A typical gallon of paint
will cover around 400 sq feet. When spraying you will lose between 15-20%.
With some rough calculations, I think I'd get a gallon and 1 quart based on a 10' long, 6' wide, 6'tall camper.
Of course, you may not be painting
the bottom thus saving you about 60 sq feet of surface. And, I didn't take anything out for the windows
. Hmm, one gallon may do this. But, I'd sure dislike running out and having a slightly different color in that quart. If you do get a gallon and a quart, make sure you mix the two together thoroughly before any application.
The color you are applying may or mayn't cover in one spraying. You need to take this into consideration, too. If going to a dark color, I'd get two gallons, plenty for two coats.
Good luck! Sounds like fun to me!