This might be old news to some of you, but I stumbled onto this guys testing of tank flushing systems. It was an interesting test and the old "Ice down the toilet" method didn't fare too well. Seems it is a waste of time and money basically. It wasn't much better at cleaning the tank than just straight water. And, as you'll see in the video, he drove his test tank around in a manner that is absolutely impossible with a large 5th wheel.
Anyway, if you are interested, here is a link:
http://www.thefitrv.com/rv-tips/test...rv-black-tank/