For those in Ontario Calgon powder can be found at most Loblaws for $7-8. I've never seen the liquid version.
I use 1 cup with warm water and a 1 cup of cheap clothes powdered detergent (cheapest I can find on sale), with a few gallons of water right after in the black tank. I also do 1/2 cup of Calgon in the gray tank too, no soap as there's plenty from the shower and kitchen sink already,
I always do this right after I dump the black and gray tanks, on my way home it gets sloshed around and stays that way until my next outting.
Works like a charm