Hi! I was a FNG here not too long ago, and now I'm a FOG. Doesn't seem to be much of a difference.
What you are probably thinking of, is that most commercially built camping vehicles have the two tank outlets connected into one drain outlet in such a way that you can flush the drain outlet flange, and the hose, with the gray water after you have dumped the black. That's simply the standard way of doing this.
I suppose you could arrange a black-tank-flushing-system with your gray water, but I'm not up on that. What I do know, is that some of the "real" campers have a "sprinkler" system in the black tank for flushing it -- I just don't know if they use gray or fresh water for that.
At the less expensive end of the spectrum, you can buy -- or make yourself -- a spray wand that you stick down the toilet and swish water around to rinse out the tank.