Starting from the front of the unit, the first (black) handle is the Master (front/full) black tank. Mike was oversprayed with copper colored paint so this was pretty confusing to start with. Also, if you have a washer/dryer it drains into this black tank. Sounds crazy but that is how it was built. It actually helps clean out this tank what with all the laundry soap, etc but you do have to dump it more frequently.
The other large (gray) handle dumps the grey water from the shower.
Inside the storage you find two smaller handles.
The black one dumps the black water from the half bath and that is the tank that the backflush services.
The small gray handle dumps the galley. I don't know for sure but I would guess the half bath sink goes into the galley.
As far as the dump pipes go, the front one is the dump pipe for the Grey water and the half bath black water. The rear pipe is for the galley and master black. As you can tell black water doesn't use just one of the hoses, but both hoses. Again, that's odd to me.
I've labeled my handles and pipes using a labeler and sticking the labels on the side of the RV by the respective handles.
When we took delivery the dealership didn't know any of this so the filled each tank separately and experimented until they figured it out.
2016 Cardinal 3875FB
2014 Chevrolet 3500HD