When I installed a grab bar in our Class C, I went behind the thin plywood wall and placed a small piece of thicker plywood (or use a regular wood like a piece of 1x4) on the reverse side of the wall directly behind the spot where I wanted to place the grab bar; it was glued to the wall so it wouldn't fall down. Then I screwed the grab bar to the wall using screws that were long enough to attach themselves to the wood behind the wall, but not too long in case you have to reach into the spot for whatever reason and not get scratched. If you cannot place a piece of wood where it won't show, then get a small piece of hardwood and stain/varnish it to match the wall and follow the same procedure.