Chances are the “plug” just has wiring to carry the 12v power to the battery bank for charging purposes. So to test your solar equipment, try by-passing the “solar ready” stuff and connecting your controller directly to your batteries, using a multi-meter to checks things along the way, such as solar output, controller voltage, polarity, etc.
That should answer some questions. Then look for wiring that should connect your exterior plug to your battery bank. If its there (connected to the batteries) “ring it out” and see if both side have continuity to the plug. Also to save yourself some trouble, check your plug for 12v and if you don’t have it there, remove the four screws holding the plug and see if its even connected to anything.
If it is and the wires are connected to the batteries, you can determine which side is pos and which is neg. to address the dealers polarity guess. If it doesn't have wires, or it does but no battery voltage, you have an answer. Just approach it methodically like trouble shooting any electrical problem, and you should find the answers.