look at the main drain pipe where you attach the sewer hose. now follow the main drain pipe, probably the 3" black tank drain, up. you are looking for tees where the drain pipes from the other tanks attach to the main drain pipe. how many other pipes join the main drain? each will come from a separate tank.
also look to see if you have a second sewer hose attachment somewhere else. from this little exercise you will be able to tell how many tanks are being drained. each tank will have its own t-handle to open and close the valve for that tank.