Using the black water tank as a second grey water tank

We had a TT with 2 40 gal. grey, 1 30 gal. black. Of course the shower tank filled before the galley or black.
To extend our stays I made up a 4th temporary valve placed on the dump, then opened the 3 tank valves which allowed equalization in them so the entire 110 gal. capacity was used.
Mixing the grey and black actually helps on dumping. The extra water in the black helped flush the solids.
Do it.