Use gray to flush black

Before I got a trailer with a Sani-flush, I would drain the black tank first...leave the black tank valve open....Then......Carefully remove the end of the hose from the dump station hole and hold the hose up in the air, higher then the tank and reach down and open the grey valve. The grey water would run into the black tank 'til it equalized...tricky part was shutting both valves while you continue to hold the hose up. And there would be a little "mixed" waste water in the hose while you got it back to the dump station hole. I suppose you could install a third valve to manage that though.