One of those clear sewer connections seems to be one way to know which tank your draining...
The Gray tank connected to the shower.... mostly runs clear looking water and mostly plenty of it... that is unless you've decided to leave it always opened when your connected to a sewer connection.
That makes the other Gray tank drain the kitchen... which in our case tends to be a little slower draining... and a little less clear looking water running through the clear sewer connection.
The Black Water tank... well.... it's probably never really BLACK Water flowing..... If it was I'd start to Worry !! But You'll soon figure out the color of that one draining through that clear connection as well....
Marv