Just a thought, but how about a "Flush King".
Click here.No installation required, won't have to disturb your underbelly.
Here's a short video how it works.
Click here.This is much simpler than the work it takes to install a flusher on your tank. I use the back-flush method with a Flush King, but I back flush from my gray tank to the black tank Dump, and repeat. Then fill the tank with the Flush King or an additional "swoosh!" amazing how much more comes out when you do this.... Best thing of all ... no risky installs to deal with.