Why?
What I might suggest is dump the tank, and then always make sure that you have about 5 gallons of water left in there. I was told that a dry tank and turn the soft "Left behinds" into a powdery sludge, that will become somewhat like mud, and difficult to wash away. By moving the trailer with 3-4 inches of water in heh tank, then that will break up the stuff, and the next time you drain it should all come out. A 10 mile drive should break it up, but a camping trip of 25+ miles is typically more fun.
Fred.
