You have a few concerning statements in your post, Like "as hardly any moisture". But any leak is a big problem in rvs. flex seal is not the way to start. First is it leaking sitting at the camp site or going down the road? Look at the dicor sealant around the cover, the rim seal on the vent itself, is the cover closing all the way? start there.
2017, 3500 Ram Big Horn, 4x4, Crew Cab, DRW, Aisin Transmission, 4:10. Curt Q20, 2014 Mountaineer 331 RLT, Sailun's on the rv and truck.