I have an older thread on the truck charging system for the trailer batteries. I had the fuse in the truck for this repeatedly blowing with no visual evidence this had happened. I finally added a self resetting circuit breaker of an amperage lower than the corresponding charging fuse in the truck. I think it was a 25 amp fuse in the truck and I added a 20 amp self-resetting breaker in the charging line just after the trailer connection box for the umbilical cable. After doing that, my truck/trailer charging system is reliable, the batteries are charged, and the truck fuse no longer blows. I think I had momentary high current draws on the charging line for some reason.
The critical point of this topic is that the trailer battery charging circuit from the truck is PRIMARILY THERE TO CHARGE THE TRAILER BATTERY SO THAT THE TRAILER'S ELECTRIC EMERGENCY BRAKING SYSTEM HAS POWER IN THE EVENT OF A TRAILER - TRUCK DISCONNECT, WITH THE EMERGENCY BRAKING LANYARD SWITCH BEING PULLED AND ACTIVATED. This should immediately apply full electric braking on the trailer and STOP the runaway trailer. Any other functions, such as running electric refrigerators, is simply extra to the trailer battery's primary emergency braking safety function.