You can cross connect the black and grey tanks if the black tank is lower (usually the case) and gain a day or 2 of space in the grey. There's a way to do it with a third dump valve or you can just hold the hose up higher than the tanks and open the black then grey. I have a clear connector so I can see when the swirling stops, then I close the black and grey and let a very small amount of grey water drain out of the hose if it's allowed.
Doing dishes in a pan and dumping the pan into the toilet is another method.