ours actually tends to fill the black first....
But we have a factory macerator (which I love....)
I will generally:
Turn on the pump and open the black and pump it until it is dry and the pump screams.
Open the gray for 30 second and then close it. continue pumping the black until the pump screams.
Close the black open the gray and pump it dry.
With our system, the techma electric macerating toilet tends to use a lot of water, we often fill the black before the gray.
If they still make these toilets, I highly recommend them....
They chew up about anything.
Nothing comes out the black tank but liquid...
We use 2 ply paper, not a problem ever....
While I've had to rebuild my sanicon once, I've never (KNOCK ON WOOD) had a problem with the techma, even it's tank level display has always worked perfect (KNOCK ON WOOD, LOTSA KNOCK ON WOOD, I SHOULD
N'T EVEN TALK OF THESE THINGS...)