Is it normal to fill up the grey water tank this fast?

Nobody has mentioned the possibility that it is the tank sensor at fault and that the tank is not really full. It happens to me all the time.

With the gray tank I don't worry about overfilling. If it should over fill it will come up in the bathtub harmlessly. I've stopped worrying about it.

When I do drain the tank I leave it open for a looooong time. Sensor still says 2/3 full. I have a backflush elbow on the drain but that doesn't clear the sensors.