I have the built-in Flusher and a clear connector. If I really want to do a good job of flushing it does take several tries. Dump then let the flusher run for 20 seconds with the Black Tank closed then dump again a look to see what comes out. When only water is visible stop dumping. Add a couple of gallons of water plus other stuff; chemicals, Calgon and Dawn and enjoy.
There is a video of the different flushing techniques which shows that a Back Flusher in lieu of the standard connector does a better job of flushing than the repeated tries with a built-in sprayer. I have one but have never used it.