We have a black tank flusher but it only does so much.
About twice a year I use a home-made flushing wand to really clean it out.
The wand is a 30" piece of 1/2" PVC with a 3/4" thread on one end (for the hose) and a cap on the other end with two 3/16" holes drilled in it. I use and old washing machine supply line to attach it to a garden hose.
The force of the water coming through those tiny holes creates a powerful jet that rips anything loose that is stuck in the tank. Because the pipe is small, I can aim it upward and all around the inside of the tank. I hold it on the sensors (wife watches panel) and it cleans them in seconds.
I store the whole thing in a larger piece of ABS.