If the vent you are talking about is the front one over the drivers seat...that's just a blank to cover the SAT TV Prep wires in the roof. The other one in the rear is a real waste water vent and has a pipe connected to the plumbing. The cap keeps rain water and debris from getting into the tanks...congrats on the fix.
It isn't always easy to see Outlaw specific posts, so I always post questions to the Outlaw "Brain Trust" of other owners here:
And there's lots of other Outlaw stuff here:
OUTLAW Mods and Easy Fixes
Kim and Steve, Mustang LCDR (Ret), '07 Damon Outlaw #1193
I have seen gross intolerance shown in support of tolerance, Samuel Coleridge
WE LOVE OUR OUTLAW RV