I added shelves to most of my cabinets. I screwed wood strips to the walls, including the RV walls. My RV walls are about 1+" thick so as long as the screws do not protrude more than about 1/2 inch beyond the strip, they will not go through the wall. The inner layer of wood is only about an eighth of an inch thick so longer screws don't buy you anything. Use lots of screws to distribute the load.
My shelves are plywood, about 3/16" thick and just sit on the wooden strips. I did attach a wooden strip to the exposed front of the shelves.
I don't know how your RV is built so try a test screw in some obscure place. Your walls are probably a similar thickness since AFAIK all RVs have about the same thermal insulating properties.
Class C, 2004/5 Four Winds Dutchman Express 28A, Chevy chassis
2010 Subaru Impreza Sedan
Camped in 45 states, 7 Provinces and 1 Territory