1 of the Mods Dad had me do to his 5th wheel was put a drain point at about the 3/4 level of the grey tank. Had 2 valves; Open 1, would drain into stinky slinky, the other thru a hose bib to fill a container, into the sewer with a small hose, or where allowed, to water bushes.
FHU site, drain top of grey until black full, still plenty to chase the black. When he had to haul all water, do dishes in pan, and dump in toilet. (Can flush with dirty water)
As for connecting the tanks. Once you do that you have no grey water, it is all black, unless the black can't back into the other tank. And that connecter will be a good place for blockage, hard to clean, and real stink to grow.