Sewer Flush - do you open just black tank or both

On my HR the factory installed tank flush is only on the Black Tank, I have a Valterra Flush King I bought that will cause a surge of water from the city water to rinse the tank, 3 min. and flush, do this a couple of times then I close the black tank valve and open the grey valve, (it is open most all the time) then I can back flush the grey tank a couple of times, you woul;d be shocked at the little pieces of junk that is in the grey tank, a couple of flushes and no smells.