Just changed the AAV under the bath vanity in our older motorhome. It was just hand tight and easily unscrewed. The new part was an exact match from the plumbing department at Lowe's (black, 1-1/2" pipe size)and cost a whopping $4. The symptoms were exactly as described in the OP, and initially I was misdiagnosing the issue as a black tank problem. I just couldn't get it in my head that the grey tank could smell THAT bad, LOL. Good luck, easy fix, and cheap enough that I would do it, just to check it off the list.
