That is an Air Admittenance Value, not a vent. But check it, it could be bad. You should also have a vent but I'm not sure where it is on your trailer. I had the same issue. Found a spider web in the vent that may have restricted air flow. Also realized I was running the exhaust fan with no windows open to let air into the trailer. This could create negative pressure in the bath area and suck grey tank air in through the AAV if it is not functioning properly.
Doing the above fixed my problem, at least for now.
Cedar Creek Lake, Texas
2016 Wildwood Xlite 171 RBXL
2016 Ram 5.7L crewcab.