I have a 2006 Montana and have had an occasional smell from the black tank. I checked the vent and rinsed it several times but would get a bad smell sometimes after flushing toilet. I had a rotating vent cover on my previous trailer so I thought this might help on this one. When I removed the three legged cap over the vent i saw there was no sealent on pipe to flange leaving a gap. The smell was going back into the wall to the bottom where it was going into the heater vent to keep tanks from freezing. Fought this problem for four years now glad to finnaly find problem.