Having lived through 2 major roof leaks I would lean toward a none sealing vent, as RayChez mentioned. The first leak we saw was also from the vent on the bathroom. When I took it apart I found that it had about 1/4 inch gap and a cracked outer frame. I removed the obstructions and sealed the crack now it seals correctly and no more leak.
The other leak, must more major, was in the roof somewhere. I started with the eternabond on the front and rear cap. The leak was 1bout 90% fixed by resealing the front and end caps. I did end up having a RV repair shop reseal the rest of the roof with the Dicor and we are now leak free. My better half is very happy
My daughter and Son-In-Law also had a lead around the skylight in their bathroom in their travel trailer. We removed the skylight and installed plexiglass cover sealed with eternabond and it doesn't leak anymore.