If you are planning to dump gray to the grass, be aware that the practice is not allowed in most places/campgrounds/etc. You can do it on your own land, but even then the practice is questionable unless it's done correctly.
They allow dumping of grey water in most US Government Bureau of Land Management parks here in the Southwest
Not sure if it would work on all installs, but it seems pretty simple to have all the gray water drains go to a common pipe before feeding into the tank. A simple diverter valve on that pipe could send the gray water to the ground before it ever hits the tank.
What I like is the pooperator like Dave shows. Who needs a dump station, just run the hose into any toilet and pump away. lol