I had a Sightseer for 4 years and this is the type of setup I use to flush the black tank.
I first dump the black tank, then connect the flush water hose to the valve assembly linked above and run water into the black tank for 2-4 minutes and then dump again.
I also close the valve in the assembly linked above and with the black tank empty and the black tank valve open, I then open the gray water valve for 5-10 seconds to let the gray water rush into the black tank. I quickly close the gray water before any of the black tank water can go back into the gray tank. I then dump the black tank again. I usually get 2-3 inrushes from the gray into the black tank before the gray tank gets too low. Of course this depends on how full the gray tank is.
I usually do this until the black tank is dumping mostly clear water.
It is a bit of a pain to do the above process, but I couldn't find an easy access to the black tank to add a flush connector.