[JRTJH]

You can start your tow vehicle (most have a relay that disconnects the battery/alternator in the tow vehicle when the ignition is off.

With the tow vehicle running, you should get around 13.6 VDC between pins 4 and 1. You can determine the location from the diagram below.

If you have voltage on these pins, then you'll need to troubleshoot the trailer wiring to determine what is interrupting voltage to the battery while towing.

If you do not have voltage on these pins, it's probably a fuse in the "high voltage fuse panel" located under the hood.