Most of us use the gray water for assisting of flushing out the waste water tube.
The extra help in the flush is accomplished by holding some water in your gray water tank, and after dumping your black tank, close the black and then dump the gray.
Gray water is much cleaner than black water and the rush of it flowing through your hose will most likely not leave you with a smelly mess in your drain hose to clean out after you've finished your dumping operation.
How much gray water you leave for that last flush is a matter of trial and error. Most would wait until the gray is almost full to get the full benefit, or just fill the gray then dump the tanks if in a camp site that has water hook ups. If you have a black tank rinse system, use it while you are draining the tank.
The object is to rinse as much debris out of your hose as possible in the shortest amount of time.
hope that helps
Dan & Loretta, US Army Retired Aero Scout Pilot
2012 Fleetwood Providence 42P Class A/DP
Spartan Chassis, 8.9L Cummins 450HP