Try using one of these to back fill the tank...it may push through the clog...that being said be careful if you build up to much pressure you can pop the seal on these....had to fix mine once because I forgot to open the dump valve
The other suggestion is a good old fashion plumbers snake to break up the clog from the dump valve side. Be prepared to take poop bath if you try this however.
I used this every time I was done camping if I had full hook up.
Basically I agree with the others....keep filling it with water and draining hopefully you can clear our the tank. It may take some time.