Ding Ding Ding!!! We have a winner. But Peggie is all knowing queen of poop (Said with respect and love). She stated:
"but your description sounds like it actually squirts, which means something has to .be preventing the flush from going through the pump...a duckbill encrusted with urine crystals or clogged with waste and TP as a result of using too little water."
What I found was a collapsed duckbill on the outlet side of the pump. All encrusted with a hard substance that is almost harder than granite!! The system is old and it is time to upgrade the pump.
Yes water is your friend!! I preach that when we have guests and they get "Potty Trained!"
Peggie is also correct that it is an S-Pump. Waiting on parts from Marine Sanitation out of Seattle.
Thanks for playing TF Jeopardy....