Should be really easy to check. Close the main sewer connection, then run some water into the outside kitchen sink. If it comes out the 2nd sewer conection you know its connected, if not check the main sewer connection and it should have water in it.
Then, I would go back to the original settings; closed main connection and open 2nd connection and run water through all of the other water outlets in the trailer one at a time; sinks, showers, tolets, etc. If water comes out from any of these others you know which outlets serve the 2nd connection.