I saw a 1970's something slide in camper, and it had a expanding back wall, that when open had tent sidewalls, and a bed in the back. The wall stayed attached at the top, the bottom swung out about 3 feet. So the solid sidewall (or rearwall as it was) kept the people inside dry.
Then in 2004, I started seeing rear beds that slid out of the back of a small lightweight travel trailer. Later I even saw some with solid walls that slid out over the hitch. Sort of a manual slide out room, but it slid out the front or back, on certain models.
Then some had tent sides, so that they are lighter, and less complex, easier to install and perhaps easy to set up? Personally I think that solid sidewalls are easier to set up and put away. You have no worries about putting them away wet and mold starting on the canvas. . ..
Have you looked at
ALiner.com They have a solid sidewall and solid roof that pop up to give up to 7' of headroom!
I wish that Aliner made a truck camper, with it's unique roof system running front to back, with 5' wide panels the length of the camper (up to about 12 or 14' long) that would fold up and meet in the middle, giving plenty of headroom above the cab.
Fred.