Before going, open the drain on the fresh water tank to empty that. Turn off the water pump (do that!), and open the hot water tap at the kitchen sink, go out and take out the plug on the water heater and drain that.
When all drained, leave the pump off, put the water heater plug back in, start to fill the fresh water tank, leave the sink hot water tap open. Watch the monitor, when the fresh water tank is about half full, turn on the water pump.
Wait till the air comes out of the hot water tap and it runs full flow--that means the hot water tank is now full. Close the hot water tap. Keep filling the fresh water tank.
When all done, empty the grey tank so the water you poured down the sink with the hot tap still open is drained, leaving you max room in the grey tank before you go.
Also now it is full and pressured up, confirm your hot water tank plug is in right and not leaking any.