I have an 18ec that we want to mount some cabinets to, saw this post, called the dealer who called Forest River. No help whatsoever. Maybe I will try a different dealer, but don't see any other options except to drill through the walls for a secure mount.
Use 3M 5200 Marine Adhesive Sealant. I use it on my boat and RV. Works Great. Use some large diameter course thread wood screws that are long enough to go through the cabinet and 5/8" to 3/4" into the wall. Use the 5200 when attaching the cabinets to the wall. When putting them up, do not tighten the screws real tight as this will squeeze out all the 5200. Come back 24-48 hours later after it cures and screw it tight, but not so tight it stripes out the screw holes.
See the factory video below. It states the side walls are 1" FRP and the flooring is 3/4".