Bruce,
My personal feeling is that you have an
unflaming pile of poo beneath the throne....
Take a hose through the bathroom window and insert it into the chute allowing the bowl valve to hold it in place..
....Turn water on high and it will begin to break obstruction into itty bitty pieces so that they will flush when you pull the black tank valve.
This will probably be more effective
Without the Sani-Con Take my word for it !!!
If you really don't trust me,
you can close off the rest of the bowl opening with a rag so any backsplash will be at a minumum...
Leave the water running until the tank is 3/4 full before pulling the valve and draining tank.. You may want to do this more than once to cleanse thoroughly...
Let us know how
it goes !!