That is an AAV....Air Admittance Valve.
Installed downstream of sink drain 'P' traps.
They OPEN due to negative pressure being created in drain line when dumping grey tank....allows air in breaking the negative pressure and preventing 'P' trap from getting sucked dry.
You have a better quality AAV then is normally used in RVs (Oatey Sure-Vent)
I took my Medication today. HAVE YOU?
Dodge 3500 w/Tractor Motor & NUWA 5vr
US NAVY---USS Decatur DDG-31