In my Newmar Baystar Sport the AAV for the gray tank is under the bathroom sink. The black tank flush back flow device is also in the bathroom cabinet but hidden by some drawers. Took me forever and a lot of DW “encouragement” to find it. As mentioned they go bad but also if you somehow introduce debris from the hose it can get stuck in the valve and not form a good deal allowing odors into the bath area.
2016 Newmar Bay Star Sport 3004
2015 Jeep Wrangler 2DR 4WD Sahara, Automatic, Hard Top