i use a pressure reducer and a backflow preventer when using my winne supplied black tank flush system. i have a clear plastic VALTERRA hydroflush adapter that makes the fluid coming out of the black tank visible. i flush the black tank for 3-5 min, turn off the flush water, and let the tank drain. if the water does not run clear, i shut off the drain valve and add water to the black tank for 3-5 min, then drain the black tank. i repeat this until there is no waste draining from the black tank, usually 4 times. caution: do not close black tank drain valve when water is draining. if you catch a piece of tp or other waste with the blade valve it will not close properly until you clean the obstruction out. this messy operation is best avoided.
after the final black tank drain, i add 2 cups clorox to the gray tank prior to draining it to clean out and deodorize the stinky slinky.