Here's the process I use when flushing my black tank. This assumes your already hooked up to a dumping station with a clear hose fitting at the drain port.
1. Close the gray tank valve and turn on the kitchen faucet and to put some water in my gray tank.
2. If my black tank isn't full, I'll turn on the hose for the black tank rinse while the black tank valve is closed. While I'm doing this, I'll stand right next to the indicator panel and watch the black tank level.
3. The moment the full light starts to flicker, I run out and open the black tank valve. I'll let the rinse keep going as the tank drains until the rinse water runs clear.
4. Close the black tank valve again with the rinse going and return to the panel and watch the black tank level.
5. Run out and pull the black tank valve as soon as the full light starts to flicker.
6. If the water runs clean, turn off the rinse and let the remaining water drain. Repeat steps 4 & 5 untill you see no more debris coming out.
7. Close off the black tank valve and open the gray. This will both empty the gray tank and rinse your sewer hose.
8. Turn off the faucet that was turned on in step one.
9. Close the gray valve once all remaining water has drained.
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