Galley sink/gray tank odor

[CooterB]

There is an extremely malodorous stench coming from the galley vent on the outside of the RV. The sink has a traditional p-trap and does not have a ASTVD under the sink. I have found a waterless trap on the washer drain as well as the bathroom sink drain and assume there is one on the galley drain, but can not see that part of the plumbing without removing some of the under 'walls'.

Is there anyone here who may be able to shed some light on my situation? I assume there is a trap, but don't know where it is or how to access it. Does this valve seem a reasonable culprit?