In a former MH that had a built in macerator I opened both valves and let the grey and blank equalize. I always found that we filled our grey 3x as fast as our black and this solved the problem. A few things allowed me to do this without relying on the single valve at the outlet. One was the built in Macerator. It acted as a valve and would not allow anything to pass unless I turned it on. It also had a bypass that had a valve and I never removed the 2 original valves and could close them at will. I also installed a tank flush on both tanks.
I think most high end MH's like Prevosts and Newell only have one very large tank. I could be wrong but that was my understanding.
I am not sure I ever understood why RV's have separate tanks unless it's just easier to install them that way. Seems like it would be easier just to have one large tank.