It also could be a faulty air admittance valve for the grey tank.  It will be inside the vanity cabinet.  The smell from a grey tank can be almost as bad as a black tank.  The valves are one way and are not supposed to let gases/odors back into your RV.
We have to replace the AAV for our galley tank which is under the kitchen sink.  It's only months old.  We're far away from home on vacation and DW kept complaining about a smell.  I wouldn't have suspected a faulty AAV that is so new.  One wiff next to it confirmed it's not working...
They just screw onto the ABS pipe and don't cost much.