To equalize the grey water tanks, I bought and installed this valve on the outside of the single grey/black holding tank outlet:
https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
It is a good thing to have, anyway, in case one of your gate valves fails or leaks, you don't get a bath, when you take off the cap.
You put this on, and leave it closed, until you want to dump. before moving on MAKE SURE the black tank valve is closed (if not, you'll have some smelly problems on your hands!). Then open the two grey tanks and leave them open. This will allow the grey tank water levels to equalize. I leave these two open for the duration of the trip, until I need to dump.
When dumping, close all of the valves, prior to moving to the next steps. Connect the drain hose. Open this add-on gate valve, and then drain like normal (black then grey tanks). Once done, I ensure all of the valves are closed, then close this add-on valve and drive home (or to the next campsite). Once I get to the new campsite, I start it all again.
Really, it works very well, is cheap to do, and doesn't hurt anything. Your tanks should be the same size, but can still work as long as they are the same height. This only works, if you have all of your waste holding tanks going to the same outlet. If not, you'll need to rig something else.
It doesn't work on my Crusader, since the grey and galley tanks are not at the same level. The grey tank is supposedly 3 feet below the galley tank (according to an email I received from Prime Time, when I asked). Therefore, the water won't make it into the galley tank.
This method would still work if your galley tank were lower than your grey tank, but would take some monitoring, to ensure it didn't overfill.