I use two capfuls of the liquid Calgon after each black tank dump. I have not had any problems purchasing Calgon on the road.
I don't believe that it does anything for smell. I does soften the water in the tank and when dumping it is a lot easier getting that clear stream to come out when flushing. It's like it coats the sides for easier flushing, that's all.
The valves seem to like it also.
Wayne MSGT USMC (Ret) & Earlene (CinCHouse)
2008 Winnebago Destination 39W
It is what it is, and then it is what you make of it.