I use the San-T-Flush system in mine to keep the black tank nice and clean.
After the "final dump" into the sewer, I close the valves, and put about a half-cup of dishwashing soap down the toilet.
Then: I fire up the San-T-Flush, and let it put about five gallons of water into the black tank.
I shut it off, and open the dump valve...
LOTS of soapy water comes out...
I repeat it; until I see just clean water coming down the pipe!
It also keeps the "Stinky Slinky" from stinking!
I hope that you can get it cleaned out and working again...