There are systems where you install a tornado system "basically a sprinkler in the black tank" and hook it to the hose at the dump station. You can dump the black tank. close the valve, fill it up with the tornado which rinses the tank, then dump again. Then the water will be pretty clean and the grey water to finish it up.
A clear hose adapter helps you see how clean the tank is.
I will clean it out several times assuming there is not a line of RV's behind me waiting to dump. The potable water is often times right in the dump lane but you have to pull forward to get to it. this way while you are filling your fresh tank you are not holding up the dump line.
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