Try it, but Dawn is useful for cleaning OIL from wildlife, depending on your diet it may have a lesser effect on human waste. I worked the Exxon Valdez spill and we used dawn extensively as a dispersant.
As a dispersant it chemically reacts with oil and makes it sink to the bottom whereas oil normally floats. In wildlife it breakes the bond to other items as it tries to let the oil seperate and eventually sink.
For a holding tank I have no Idea how it will work, but will offer one point of caution Be prepared for bubbles and foaming, you could end up with a trailer full especially if you have a black water flush.