Here is the diagram for the layout of the grey and black tanks IF YOU HAVE the "full" (rear) bath. This is from the 1977 Service Manual.
Both tanks are directly under the rear bathroom, the grey on the curb side and the black on the street side.
They share a common 3" outlet pipe, each tank having its own "guillotine-style" valve with which it is opened or closed. The valves can be seen in the rear "trunk" from outside the trailer.
It is recommended to keep the black tank's valve closed at all times, and only open it to dump the contents when the tank is full (or when you are storing or winterizing). The grey tank can be opened after the black one has been drained in oder to rinse out the pipe and your sewer hose.
Are things becoming any more clear?
ps: here is the 1977 fresh water holding tank. It's ahead of the axles.