I have had some bad smells coming from my gray tank at times also. Do you allow the tank to fill to a good amount before draining? If not food particles will settle in the bottom and rot. I will from time to time add some black tank chemical to the shower drain and run some water in the tank before traveling to help it break up any crud that may be staying in the gray tank. You could also pour some bleach down the shower drain to help get rid of the smell at that location.