By the look of all the drain valves, it seems like you can equalize your grey water tanks pretty easily.
Valterra T58 RV Twist on 3" Sewer Hose Waste Valve | eBay
You could buy this valve that goes on where your sewer hose hooks up. Using that valve to equalize/combine the tanks.
To combine them, you would keep the new valve closed. Keep the black water valve closed. Open the general grey water tank valve, then open the washer grey water tank valve.
Then you would essentially have one large grey water tank.