Welcome Al,
That connector is called an SAE automotive plug. It is typically used by Zamp to connect the solar controller to the trailer batteries. If you trace the red wire back from the connector, there should be an in-line fuse on the positive side. The wire then connects to the resettable fuse where the positive lead from the battery is connected. The black wire should go to the common ground point.
This set up is made for use with a portable suitcase type solar kit.
https://www.zampsolar.com/140watt-portable
Real easy to get solar set up quick. Think of this arrangement as a solar battery charger. This is the set up we use except we have a 200w panel.
Using the portable kit eliminates the need to mount anything to the roof and mess around with running wires.