Pump grey tank into black tank?

There is a member on the Sportsmobile forums who did exactly this. He put two check valves (one on the grey water tank, one on the black, so it couldn't go back into the grey), attached a pump, and when his GW tank is looking full, he presses a button and water gets moved to the black tank.

There is always the manual method of a five gallon bucket and moving grey water to the toilet that way.