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We have 2017 model 3921 FB and we have one discharge connection, but two grey water tanks and two black water tanks, both feed into the same drain. We also have two water connections to wash/flush the black tanks I usually drain the black tanks first then drain the grey water tanks second, while having the drain valves closed on both black tanks , I fill/ wash the black tanks while draining the grey. This way the black tanks usually flush mostly clear when I drain the second time.
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