I think you're dealing with a clogged check valve or the sprayer inside the tank is clogged.
I'll just tell you where the check valve is in our rig. We have a 2000 Winnebago Adventurer. The check valve is behind the mirror on the wall by the dinette. (When the dinette slide comes in... it covers that wall.) That would put it behind the knobs in the shower in our rig.
Seems we never thought about putting a SEDIMENT water filter on the hose when we are flushing and rinsing the black tank. We were camped in northern California and we KNEW there was a lot of sediment (sand) because we could see it build up in the mesh screen of the sediment filter on the fresh water hose.
That sand clogged our internal black tank sprayer.
p.s. Don't try using air pressure to unclog it. ... just saying don't.