Do you mean your black water drain is plugging? If so, I would suggest you do what a lot of us do and that is to not leave any of the dump valves open all the time. Wait until your grey water starts to get full, then dump the black, and follow up by dumping the grey. It flushes everything out and keeps things running smoothly. You'd need some sort of smooth PCV pipe to be able to leave the valves open all the time.
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