Gray tank plumbing vents

[RVific]

My Ventana gray tank showed 100% yesterday and the only evidence was that the kitchen sinks wouldn't drain. I expected that I would instead see water in the shower base first but that was not the case. Previously I had noticed the kitchen sink gurgling when the washing machine drained and the grey tank was maybe three quarters. So, is there something wrong with the vent under the sink. The vents seem too simple to be broken and they're glued on as if they would never need service.

[Skip426]

First ; as per washing machine instructions ; grey tank drain valve should be open when washing machine is in use.
The air admittance valve on the plumbing isn't a true vent or you'd smell gray tank odor all the time.
On newer model plumbing systems there maybe an, anti back up valve on the shower P trap , but on my 99 the shower pan will fill when the grey is full.

[Old-Biscuit]

Your profile (signature not showing) shows you have a 2019 model
Suspect your shower drain uses a Hepvo type drain trap which is a collapsible membrane that allows water to flow OUT but stops water/odors from coming back in

https://hepvo.com/


As for that Air Admittance Valve on sink drain line
Those are installed after the 'P' traps and are removable...they screw into a stub pipe that has a fitting glued on
They OPEN when dumping grey tank to prevent 'P' traps from getting sucked dry from negative pressure in drain line
Cheap ones are just a flapper held in place by a stretchy tang that wears out and doesn't close ....grey tank stink inside RV
Better ones use cone shaped discs & springs

Probably have one inside bathroom sink cabinets