SOP:
When dumping at a CG upon departure.
1.Dump black, dump gray
2.Fill up black tank to top with fresh water and dump black again.
3.Fill up black tank 1/3 with fresh water. add chems and 'leave' in the tank.
Yes it's just a holding tank but it's a holding tank beneath where you sleep, eat and live. I make sure my tanks are clean. Keeps the gauges working also.
BTW: The way you fill up your black tank with fresh water?
Find out which sink dumps into your black tank and simply just run the faucet from that sink.
You can check how full it's getting, (until you figure out how 'long' to run the faucet) by stepping on the toilet pedal and opening the blade. You can see the water level when it comes to the top. Which btw also you need to get clean water up that far to rinse clean that area also often.