For what it's worth, some small Airstream trailers have one combo tank for black and gray according to specs.
I don't know how they flush the hose after dumping. Would be interesting to find out if they have some other provisions like external rinse, or if it's just a little messier.
Granted, it's not exactly the same in that the single tank may be easier to access for cleaning than would be the gray tank of a motorhome if "stuff" backed into it unintentionally.
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