from experience, here is what the problem is. There is likely a hole in the rubber roof which is letting water in, running down the wall(s) and settling in that area which has rotted the wood from the hole to the puddle. Once it has settled in a puddle it has rotted the wood that the fiberglass is laminated to (delamination). This can be repaired but will take a lot of time. If you do not have carpentry skills I would have a repair outfit fix it but be ready to take a good hit to the wallet. I fortunately have carpentry experience and friends that have built campers for guidance. I would show you pictures but cant figure out how to do it on this sight.
