Hard to say without some context. Generically, the "skin" is whatever material forms the surface layer of the RV exterior, be it fiberglass sheeting, gel coat, aluminum, or whatever. If the skin gets damaged, it can be removed and replaced, or in some cases repaired in situ using standard automotive body filler techniques followed by re-painting.
Adding to Gary's response, when we (I) managed to create deep gouges in a couple of bay doors, the repair shop "re-skinned" them i.e. they replaced the aluminum skin on the doors rather than put on new doors. After painting, the bay doors looked as good as new.