Absolutely get one of the above vents for the black tank. They DO work. As far as the vent cover not being able to fit because of the proximity to the black tank vent, you have several options: There is a lip that goes around MaxxAir vent cover. If the black tank vent pipe is rubbing up against the lip of the vent cover, trim it. The other option is one you may not like as much. Return your Fantastic Fan and exchange it for one that has the automatic rain sensor on it. That way, you wont need a MaxxAir vent cover at all. It will cost you though.
How close does the black tank vent pipe come to the existing vent opening that you want to cover? I'm really thinking you can do a trim job on a MaxxAir cover. Its been done before. I hope this helps, and sorry for that man, I know its very frustrating.
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2009 Eagle Superlight 303RKS, Hensley Arrow, 4 sets of Bal stabilizers to make the wife happy, and 2 Yamaha EF2000is inverter generators. Towed by a 2013 Toyota Tundra 4x4 CrewMax 5.7L.