First of all I would check the pilot lights/stove knobs on the stove. I had one bumped on and thought it was the gray tank smelling. Turned out to be propane coming out from a stove knob that got bumped to the on position and the propane was finding its lowest spot behind the stove and into the sink.
Are all your p-traps filled with water? That water can evaporate and allow smells up thru the drain.
If all that checks out ok. It could just be a stinky gray tank. Yes, they can smell awful and fill the RV with a smell. Depending on all what food stuffs and grease you allow to go down the sink and how often you clean it or don't.
I pour one quart of lemon cleaner found at any grocery store every other time I dump the gray tank. Quart of lemon cleaner is around a whole dollar at Winn Dixie stores.
