Wiring a charge line to battery on utility trailer

I would change the connection to a 7-pin standard RV plug. As was stated use a fuse on thge (+) line by the battery. Personally I would go with #10 wire and 30a fuse. Also consider a relay of some sort so that the charge line to the trailer only has voltage when the engine is running, otherwise you have the potential to drain the truck battery with winch usage while the truck is parked.

Going to a 7-pin now also makes it easier to add brakes in the future if you ever get a heavier trailer.