How old is your RV? Any chance you are smelling ammonia from a fridge that has just begun to fail. When cooling units rust out internally, you get a rotten egg smell.
I've also had the sulfur smell come from our hot water tank, but that is only when using hot water. Something with mineral build up, but it always goes away.
Sure hope it's not your fridge off gassing. Open the outside vent area and sniff around. You'll know quickly if that is the issue.
I also know gray water tanks can get almost as bad at the black tank, and many RV's have a tank vent right under the counter next to the sink, instead of a stack that goes out the roof. Could it be the check valve in the sink vent is stuck open?