On our 94 Land Yacht (Diesel Pusher), viewing form the drivers side, (looking in the open hatch door), the Grey water comes from forward (Front) and the black water comes the the rear (Back). They share a common dump line with seperate slide valves.
We always dump black first, then use grey to rinse. It also has a water inlet to use to rinse both tanks, plus a drain (or shower faucet that allows us to use our own water to rinse by looping a short hose from faucet to inlet)
"The Roadhouse" our Land Yachts name