X2 We had a similar problem with an older fifth wheel we picked up cheap enough to make repairing it myself worthwhile. I carefully removed the trim and aluminum skin until I reached clean wood. Use a thin putty knife to clean the sealer out of the lap seams before trying to separate the panels and try not to damage the lip or you will have a fight reinstalling them. Once the rotten wood was removed I was able to reconstruct it with a table saw and basic hand tools although a miter saw would have sped things up. A few dry weekends, about 500 bucks for materials and tool rental and we have a serviceable trailer that we really enjoy.
