Using a hose to back fill your black tank wastes an unGodly amount of water.... if you do this several time.... omg....
what I do is drain the black tank first. then, Lift up the drain hose and open one of my gray tanks (which are usually very full), back flush the soapy sink water into the black tank. When water levels are even (halve in each tank), shut the gray tank valve and lower the drain hose and flush the black tank. repeat and rinse... repeat and rinse with other gray tank... no wasted water from hose.
I also have an 'auto' black tank flush connection if I think the tank is not clean enough, but I have only used it once to play with it.