I sent you an e-mail about a year ago regarding a leak I had in a 27.5-foot Forest River Cherokee fifth wheel. Here is an update on what I found and how I fixed the problem in the event you hear of other people who experience this.
I opened the back wall on the inside of the trailer and discovered (and removed) wet insulation. The area around the tail lights was dry with no water stains but the 2×2’s in both corners were black. I then went to the outside of the trailer at the rear and removed the trim that covers the screw heads along the top where the roof is radiused. There were drilled holes for screws at either end with no screws in place. If water got in behind that piece of trim it would run into those holes and down the inside of the wall, exposing the 2x2s to moisture and causing the black mold.
After finding this, the 2×2’s were replaced, screws were put into the empty holes and caulking was used to seal it. Since then, it has been dry. I would like to thank Ian Mundy for his article on leaky tail lights and thank RV Lifestyle magazine for publishing these articles.