The biggest thing preventing a "poop pile" is using lots of water when flushing, and ensuring plenty of water in the tank. The water used when flushing #1 helps keep things floating in the tank preventing #2 from piling up.
By only using the tank for more "important" things you didn't put enough water into the tank and now have pile instead of a slurry (sorry sounds gross but the best I could to to explain what happened)
Fill the tank with water and drive around a while then find a dump station, hopefully things will break up and go down the drain.
