no need to add a cassette toilet to do this. If your tanks have a common outlet (very common), get a twist on gate valve. Then do the following
When the grey tank is full:
1) attach the gate valve to the dump valve, CLOSE THE GATE VALVE.
2) Open the grey tank valve, then open the black tank valve.
This will backflush grey water to the black tank and equalize the volumes.
3) once the tanks equalize, CLOSE both the grey and black tank valves.
you can continue to do this till both tanks are full.
4) when ready to leave, make sure both the grey and black tank valves are closed and using a small bucket at the twist on valve, open this valve to flush the gallon or so of water in the dump line and dispose of it.
5) remove the twist on gate valve.
I've done this for years. And actually I also do this when I dump easy way to backflush the black tank at a dump station.
