Do you have a volt-ohm meter? The one(s) with almost 0 ohms resistance (with the meter set to measure ohms) when you touch one probe to the terminal and the other probe to exposed metal on the RV frame is/are "-".
Sometimes you can just trace the wire to the frame as well. Anything wired directly to the frame is "-" on the battery.
Also on your unit you can almost certainly run the converter without a battery. If the terminals are securely and safely in a place they will not touch each other or anything else (this is really important to avoid damage), you can find +13.8v or so with the converter on briefly using a volt meter with the black probe touching the frame, and the red probe touching each terminal one at a time. The one powered by the comverter to +13.8v will go to ""+" on the battery.
Jim
