Our rear kitchen 5er had the smaller grey tank at the back with its own dump valve that led to the main dump valve. The big grey tank up closer to the bathroom had its own valve.
I confirmed the amount each tank could hold individually using buckets at the dump valve after filling the tanks by running taps. "Full" is when the water comes up into the kitchen sink for the galley grey and up into the bathtub for its grey tank.
I didn't like to mix the two tanks by leaving valves open because the galley grey water is nasty, while the bathroom grey is clean soapy, so it is good for being the last to dump and clean the sewer hose.
While off grid, the galley grey always filled first, so I would have preferred it be the big one.
