I would make sure all of the sink traps have water in them. I might also add a ziplock bag of water over the shower drain just to make sure that no smell comes from there.
Keep in mind that some times the smell is actually from the gray water.
The bulge and the smell may or may not be related.
I added the 360 siphon vents to my tank vents to take care of what I would call the occasional smell. They work as advertised.
1998 Tioga Class C
400 amp/hrs solar, Trimetric
Retire 2020 GLW