You can do the inside wall the same as the exterior using 1/4" finished plywood. What I did was measure and cut my piece ( be sure to one solid piece as oppose to several smaller ones. This will add more strength over all). Once my piece was cut and tested for size, I applied a thin coating of epoxy resin to both sides; again this strengthens the wood. Let it dry and air out for day or two. You can install this against the interior foam and beams with screws and liquid nails or PL construction adhesive or any like bonding glue. I do not recommend applying epoxy inside the RV, it will stink for weeks. Even the exterior resin odor will seep inside. You'll need to air out the rig with windows open for several days to remove this smell. Allow for 24 hours curing time for the construction adhesive to bond. Once complete, you can paint or paper the wall as you see fit.