We had a poop buildup problem in our 2008 fifth wheel. I attached a nozzle to a length of hose and adjusted the nozzle to the highest pressure flow. I pressed the foot pedal to open the toilet and ran the nozzle down into the tank. With someone standing by to turn on the water (gradually, of course), a bit of work with the nozzle did the job of breaking up the pile.
I THOUGHT that I had been using plenty of water as recommended, but eleven seasons of camping was enough to form a blockage.
I threw the hose away, of course.