Water intrusion is a RV's death sentence, but most water intrusion is preventable. You will find almost all trailers will have a rubber roof. Careful inspection of all areas where the roof meets another material is critical. Roof vents, sides, and the area where the roof and front and rear sides meet should be carefully inspected at least annually. A pin whole in the caulk can cause significant water damage. Windows, doors, and clearance and tail lights are another source of water intrusion. You are right that a fiberglass bonded sidewall is difficult if not impossible to repair if damaged by water intrusion,