Hey JDOG I followed your camper build on youtube with interest!
I'm researching boat building at the moment as a newbie as well. For building the walls I think you should first decide if you want insulation or not.
If not, I would look into "stitch and glue" with 4mm plywood (1/6") with some light epoxy and fiberglass and use a curved shape for stability. With 100g fiberglass and epoxy on both sides (1 oz) that should be something like 108kg (240lb?). Probably twice that with painting and seams and ribs for stability.
If you do want to use insulation foam I'd try to use it as a supporting structural element like SIPs do. Maybe you could reduce the plywood size to 2mm, no clue where the limit are for impact resistance.