AStravelers said:The typical 7 pin round connector that connects the tow vehicle to the trailer has a +12V lead. You will need to check the tow vehicle or trailer owners manual to find which pin. It also should be labeled on the connector.
This +12V lead does supply some charging to the trailer vehicle. However because of the length of the wire from the alternator to the trailer and the relatively small wire size it doesn't do a very good job of charging the trailer batteries. If this is a truck the wire size to the connector is probably #10. In the trailer it may be smaller.
If you only have 1 or 2 batteries, group 31 or smaller a pair of + & - wires of #6 or #4 gauge will do a better job. These wires need to be run from the alternator all the way to the batteries with a heavy duty connector at the trailer/truck interface.