[jcus]

With one tank have never needed a flush out type of cleaner, there is enought water in the tank that none of the solids will stick to the bottom. On my Newell, gray water will go straight overboard if the final valve is left open, no switch necessary. If closed, gray water will go into combined tank. Fill tank with a combination till about 1/2 full, then open final valve to let gray water go directly out.  Probably not very pc, but have used my macarator to pump tank out into a field away from any people.  Used a package of black tank treatment before pumping. No smell, no solids, and on a hot sunny day is evaporated and gone in a couple of hours. Best done while dry camping way out there.