I rehabed a slide-in camper some time back, 1997 or so. Those FRP panels were great, Used them in the tiny wet bath, and also on the outside underneath area that slides into the truck bed. Fortunately the rest of the interior walls were in good shape sd was wiring etc, so did not have to go to the extent you are going through.
You mentioned the cabinets you could not pull. I found some cabinets, in my case the overhead babinet that the front dropped to become a bunk was solid in there. Some items are screwed in from outside before the outer skin goes on. In my case, it was pretty much coming out, so a hacksaw bplade inserted between it and the wall made short work of the screws.