[RMcNeal]

As Mark said, I usually dump blank tank first and flush well, then both grays.
I have a hose meter so I can tell how much water I'm putting in. I open the black tank flush and clos off the black tank drain valve. I can meter 5-7 gallons of water into the tank. Then I shut everything down, go inside and add my Unique black tank treatment. Sloshing while travelling helps a lot, but the key is to never allow the tank to dry out.