I took a 7 week coast to coast road trip after being laid off from Boeing. I didn’t have a camper but realized that being able to pull off the road at a rest or truck stop was the way to go to make the trip affordable. With a few sheets of 1/2” plywood and misc. hardware this is what I came up with. The effort was well worth the time and materials.
My best friend took a hiatus from work and traveled the country in her truck + camper shell as well! Her mattress was on a platform, so that she could stow her belongings below. I slept in there with her for about a week! It was SO fun, and I hope that when I finally get my shit together and trade my car in for a small truck that I can do the same thing!