There are quite a few threads on this issue and it is not limited to Forest River either. I have a 2014 Prime Time(which is FR) and between June and September of this year, I got the same issue. I have called Prime Time and because it is out of warranty, they will not address it. I kept the unit waxed, Aero 303, etc numerous times a year. It is a defect with the cap and gel coat. Painting will correct it for a while. Hope you get a new cap.
2014 FR Crusader 315RST
2014 Ford XCabSD F250 6.7L