I thought I would make note of Sandie's original post. Sandie specifically stated Calgon Water Softener (CWS) should be put into the black tank to make it empty better - she is 100% correct. Before working for E.I.DuPont, I worked for Calgon Chemical Company. Calgon makes MANY different products. Some products have surfactants (which lower surface tension, i.e. detergents, wetting agents), some do not. CWS works by reducing the hard elements that build up such as calcium, magnesium etc. So using CWS softens the tank contents and any detergent (Dawn, Ivory etc.) should make the other stuff leave easier. Repeated use should be beneficial and accumulative. The important thing is - use Calgon Water Softener (CWS), no other Calgon product.
Here's another little tidbit, pick up any powdered detergent in the super market. Feel the weight, the heavy ones are less expensive. This is because the makers put salt in as a main ingredient and salt is heavy. The more salt the less it cost to make. The current CWS formula has 40 > 60 % salt. The original formula was mostly baking soda & sodium phosphate, it worked much better - the original CWS box was also lighter in weight. However, sodium phosphate is not good for the environment and the formula was changed. In the future Calgon may change the formula again and it may not work as well, keep this in mind. Happy RVing