I have a Dutchman type camper with "fiberglass" sides. Most are built with a skin (and I mean thin) layer of fiberglass glued and pressed on to a luan type backer (which is wood). If moisture (humidity or water) gets to the wood the fiberglass pops off in what they call delamination. If you get delamination you have lost thousands in value of your RV.
A few use ANSDEL backer for the fiberglass, which is water resistant.