Our black tank flush was installed at the factory with a back flow preventer pre-installed. Our toilet sits directly above the black tank and the flush is just below the bottom of the toilet. I don't really think it would be possible for contaminated liquid to flow back into the flush outlet unless the water level was above the outlet.
Like Dennis (Executive), we usually leave the grey tank valve open all the time, until the day before we expect to empty the black tank. That way the grey water can rinse the sewer hose. Unlike Dennis, I let the black tank fill pretty full. I can tell by the sound of the black tank flush when the tank is almost full. Once I hear the change in the sound, I open the black tank valve. Depending on the water pressure at the park, it may take 5 to 10 minutes to fill. Just don't walk away and forget about it.
Our black tank flush sends water in all different directions, but I don't think it is enough to really rinse the tank. That is why I like to fill it up and then dump it again. The suction created by the water rushing out of the tank helps pull solids out of the tank. I usually rinse the tank 3 times until the water runs clear. You can get a clear sewer hose extension that is straight, with a 45 degree elbow or a 90 degree elbow to keep an eye on what is coming out of the black tank. I don't use any chemicals or enzymes in our black tank and don't have odor in the camper.
After the black tank is fully rinsed to my satisfaction, I put a few gallons of water back into the tank before turning off the supply to the black tank flush. I do use a grey water hose that is grey and is stored separately from the fresh water tank.
-Michael
