We, too, use the Happy Camper (
http://www.5starhappycamper.com/) holding tank treatment in our black tank, along with a capful of liquid Calgon that we stir up in a toilet bowl full of water before it goes into the tank. So far, so good, and we really like that the Happy Camper stuff doesn't have stain anything or have an odor (and neither does our black tank!).
Carolyn