I have been battling the sewer odor for 5+ years in my class A 2008 Winnebago Voyage. Sometimes it was there and sometimes it wasn't. Several RV mechanic's and several emails to Winnebago yielded ZERO results, Either couldn't duplicate the problem or locate the source. Put new "mission couplers" on the black and gray outlets with some OK results but did not solve the problem. Bought a video display unit with a 3' camera lead on it. Looked and looked inside and outside the tanks and could not see a problem.
I finally solved the problem with a little insistence from the wife. In the hallway between my bathroom and the galley and below my control panel was a wall mounted vent screen to provide air to the mail wiring chase, after removing the screen and moving all the wires inside to one side and low-and-behold way back in the corner was a sewer vent pipe. Grabbed the pipe and it seemed a little lose, put the camera down there and saw the top of the black tank was extremely dirty, twisted the pipe a little and there was the inlet for the vent pipe to rest in and I could see inside the tank. I guess I found the problem. I had to remove a small section of paneling which was very easy to do so I could get good access to the pipe, Put some sealer on it, shoved it back into the 1 1/2 rubber grommet on the top of the tank, secured it with 2 clamps that I attached to the framing of the wall, reinstalled the small piece of paneling and the corner molding (which came off way to easily) You would never know today that I used to drive the Sewer RV. It could get very smelly in there while on the road! Hope this helps someone down the road.