When I had to replace my black tank and all the plumbing because of a tire blowout, I installed a 1.5" crossover tube between the B & G tank w/ a ball valve in the center. I installed it for when the grey tank is full, and the black tank is 1/4 full or less, I open the equalizer valve for a minute or 2, gives room for a couple more showers. And that way, both tank are full when dumping.
But I also found it useful for after draining black, I open the equalizer valve for a few seconds, just to rinse the bottom and the gate valve of the black tank. Then drain grey to rinse the hose and all.
Need it or not, it feels good :)