I typically use whatever water is left to fill the black water tank.
The trip to the dump station splashes it around enough that it cleans it to my satisfaction.
If I am really concerned I will take a hose and fill through the toilet to ensure the tank is full before dumping.
Small bit of advice...
Dump your black water tank first...then your grey...the wash water flushes the crap out of your dump line...Then rinse the grey out of the hose before storage.
If you keep your valves closed, run a bit of water into each to ensure flow when you do dump.
Personally I would close the valves and fill to half full to ensure it drains properly.
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