Woodchuck,
When PC went from the original macerator to the newer Turbocon, the ability to easily transfer from the gray tank to the black tank went away. The old video no longer applies.
Don
??? 
I did not know of a design change that prevents the blending of the waste tanks. I would like to know when the change was made and also the differences.
I hate providing instruction that does not apply. Sorry Woodchuck for misleading you.
Ron,
My 2015 was one of the first to have the turbo macerator. In fact, the turbo macerator on mine is an engineering test unit. In any event, there's no crossover other than inside the macerator itself. Right after we got it, I tried to move Grey water into the black tank. Didn't work. I used what you described, open gray valve, what, open black valve. I waited about 30 seconds, closed black, closed Grey.
I saw no difference in the levels, the next morning, same on the levels. I got to the dumpsite, dumped black, dumped Grey, looked at the levels on the panel. The Grey water level showed only 1 led lit. The black took a while to show empty... which is typical for black tanks.
Never tried it again. When I stopped at the factory for some warranty work, I asked Bob about that, he told me nope... Can't do it with the turbo.
Also, for JSanford, my 2015 2551 has a "19" gallon Grey water tank and a 26 (?) gallon black water tank. No idea on newer models