Originally Posted by tomx
I noticed there is a black nob on the roof of my 2006 Roadtrek 170 popular. Does anybody know what it is for? Thx!
You can see the samll black nob on the left side of the picture attached. I think it is right above the bath room.
Judging by its location above the commode and by its appearance, it must be the top of a vent tube for your black water tank. When you flush and water goes in, air has to come out. The cap on top prevents rain from entering. Unpleasant smells are blown away as you drive, ventilating the black tank and reducing odors.
BTW, your sink - and by extension your grey water tank, also must be vented. To reduce holes in the roof the vents for both grey and black tanks are often combined.