The gray vent is tied to the single vent pipe that you can see. The tie point is in the wall behind the shower and the bedroom wall.
There is also an air admittance valve under the vanity sink.
As far as the chugging, I experience the same thing. My guess is thats it’s from the tank flexing.
Also it is a good idea to run a bit of water into the sink traps after dumping. When you dump the water in traps may be sucked out, that can result in a bad odor if you do not add water to the traps. The gray tank odor can smell as bad as the black at times.
2021 Newmar Baystar 3401
2019 Jeep Cherokee Trailhawk toad
2017.5 Forester 3051S Traded 7/2020