For those of you who must go there, here is a what we use in our Gray and Black tanks on our 2002 Pace Arrow motorhome. get a empty plastic gallon jug and add the following.
1 Cup of Pine Sol
1 Cup of houshold Ammonia
1 Cup of Fabric Softner. (snuggle or any other brand)
8 oz of Arm and Hammer Baking Soda.
Put it all in Empty Gallon Jug and add water to fill Jug.
Shake well and add one cup to each empty tank pour some down each drain. as tanks get half full add another cup. It stops Odors, Lubricates and cleans tanks and valves.
It works great for us and we have never had any smell or stink even when dry camping for two weeks in Death Valley when it was hot. That should take care of the problem.