Am going to tomorrow to get the 1.5" foam & Monday will pick up some of the 1" foam. The prices are too good to pass up even if it means some cleaning & more gluing. Plus I'll feel less guilty about using non-renewable resources if it's recycled material.
This is just the first camper, sort of my test camper, and will be made to slide onto a borrowed, 48"x72" landscaping trailer. Since it is small & has side rails I will be building it smaller on bottom & larger on top, overhanging the camper on all sides. Exterior dimensions will be approx 47"x96" on bottom, 58"x96" on top. A couple pages ago I posted a pic by GPW drew with a good example of what I want to do. Will post again as reminder.

I do want the floor to be insulated as well. So floor & sidewalls on the lower section will be a sandwich of plywood-foam-plywood.
We're not going to be going off-roading but the camper needs to be basically self-supporting since it will be taken off the trailer between camping trips. It will probably just be set on cinder blocks when not on the trailer.
What thickness plywood do y'all think would give optimal strength to weight ratio? Here's what I was thinking for the lower section:
Floor: sandwich of 0.25" ply - 1" foam - 0.25" ply with 0.25" ply furring strips used like little floor joists
Lower side walls: sandwich of 0.25" ply - 1.5" foam - 0.25" ply with 2x2's on edges. (not actual 2x2 but modern day 2x2)
Upper walls: 1.5" foam with canvas exterior skin & 5mm underlay for inner skin
The entire exterior, even the bottom, will be wrapped in painted canvas. The exterior bottom, lower wall & underside of shelf will get a double layer of canvas as additional wear & tear from will be expected. Paint touch ups & canvas repairs of that area are anticipated but care should hopefully make that minimal.
So? What do you think?