Sewer Tank Flush

I had the same problem. Tried bypassing the backflow preventer, still didn't work. The problem was a plugged nozzle in the black tank. It had a cylindrical screen that had gotten clogged with fibrous gunk. I cut the screen off & soldered on a small piece of 1/4" tubing with the end crimped slightly to form a nozzle.

Now it works far better than it ever did, even when brand new. It was actually a pretty easy repair.