I have a Valterra shut off valve that attaches to drain pipe. I keep it and the tank valves closed. When the gray water tank is full, I open the gray water then the black water. Since there is more gray than black water, it flows into the black tank. When the water level is equal, I close the black valve and open the Valterra valve and drain the gray. Close the gray and Valterra and go through the same procedure again when the gray is full. When the black tank is full, open the Valterra valve then the black valve and drain the black. When empty close Valterra and open gray and let it flow till water level is equal with the black tank and then close gray and open Valterra. If you have a clear plastic fitting you will see solids (which seem to be lighter than water) flowing out at the end of this step meaning that the solids are still on the tank bottom. After about 3 transfers from the 60 gallons of the gray water tank, you probably will not have any solids flowing out. It is much faster than filling the whole black tank to drain again just to have the solids remain on the bottom.
Gary 05 AMB DST