"Budget Camper is 13 ft long, fits on long-bed full-size pickups, and is rugged enough to take the beating of off-road trails - the kind of punishment that would reduce most campers to rubble. The secret of Budget Camper's strength is in her unique construction method. Campers are usually built up of many short lengths of 1 x 2 wood, which are held together with staples. Budget Camper's frame is different. It is built with an integral truss-style frame, cut from sheets of 4/3-inch plywood using a portable reciprocating saw. The framing for each wall section is cut from a single sheet of plywood. This creates a structure that is many times stronger, and it also means that there is nothing to come apart when the going gets rough."
So says the blurb at this site, which sells build-it-yourself plans for $45. It looks intriguing. I've always liked pickup truck campers, but most of them tend to be either very expensive, or structurally unsound. Note: if there is any sign of past or present water leakage, it is probably structurally unsound. Building your own is a possible way around both problems.