What I am doing now to make use of both tanks is I put twist on shutoff on outlet, they after a couple days I slightly open the grey valve and then the black tank, that way the two tanks can level and you can get max use out of tanks. Chances are you will get more water into the black tank which never hurts. You could just then use the usual stinky slinky and dump, but I connected my macerator and pump it out. No mess that way.
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