I would probe the wire (usually blue I think), with the controller held all the way on, each side of the connector.
If you have battery voltage one side but not the other, it's the connector.
If nothing each side, it is the feed. Check the fuse. Possible broken wire along the way somewhere.
If voltage both sides, follow the wire and check at connection intervals. Or crawl under and probe at the wheels.
Try putting a ground jumper between the truck and trailer to a good solid metal contact area.
Somewhere in there the 12V is not getting through. By following it with a probe, you will see where the broken connection is.
Check for evidence of rodents. They like to chew wiring.