I've seen a clear plastic coupling that goes between the black hose and the unit at camping world. A green hose is attached to flush the tank. Do these things work very well, or they a waste of time?
There are 2 flavors of these adapters. One shoots water back up the drain pipe. It works ok, as noted, depending on the pipe geometry. The other type has another valve on the outlet, which allows you to backflush the tanks. This is a bit more universally useful, but I liked both methods on my trailer. So I made my own which combined both features. Worked super.
Now that I have the motorhome, I find the in tank flusher to be far superior to any outside flushing method.