Install cassette toilet, use black tank for grey water??

My Dad had me add a extra line and a couple of valves to his grey tank. Open one valve, and the top forth of grey tank could drain into the stinky slinky. Close that, open another, and the top could drain thru a hose apron. Mostly he used it so he had plenty to chase the black, but sometimes would catch a bucket full and put it in the black.
Without a sewer hookup, he would do dishes in a pan, and MT the pan in the toilet.