I tried the black tank flush on my last trip....I have a new GT 328. If you just plug the hose in, have the black tank valve open, and turn on the hose, the water comes out in a steady stream, but without the large "swoosh" that you get when you open that big valve with a full tank. I closed the valve, let the tank level build up, and then opened the valve. I did that a few times, then I turned off the flush hose, closed the black valve, and opened the grey valve. I don't know if I did it exactly correctly, but it seemed to work.