My wife and her brothers did it when they were kids (early 1970's). My kids rode in the back of our pick-up truck with a shell, many times (late 1980's, early 1990's). I haven't let any of my grand kids ride in the bed of he pick-up now though.
Unfortunately, as stupid as our world has become.... kids riding in the back of a truck camper is probably not a good idea. Some fool-brained nibby-body would probably call the police and say they are locked in and it too HOT in there! It's a stupid world we live in these days. I don't advise it!