I built a clear black tank and tested the Sewer Solution

the only benefit of the ice would be if you have very little water in the tank and it can slide around and knock into piles of "stuff" on the bottom of the tank, knocking them loose so that they can drain. If you have ice floating in the tank half full of water, all you will have is cooler water.
I have read where some people, Darwin specials, have put marbles in the tank to accomplish this. then there is the issue of having them roll around the next 2,000 miles until they all drain out. :)
bumpy