Leaking around the vent fan is not only common, it is almost inevitable, and very likely stems from an install issue at the factory.
Take it all off, down to the roof, to see what the cause is, then replace and recaulk.
I could describe what Doug found when he removed ours, also the leaking black tank vent, but it is too early in the morning to get my blood pressure up.
Fantastic Fan is great to deal with, and will promptly send you any replacement parts you need....then walk you thru it while on the roof, if that is needed.
Good luck.
Maggie