It probably isn’t worth charging the trailer from the truck. The truck would be capable of towing with an empty trailer battery anyway. Burning extra diesel to

It probably isn’t worth charging the trailer from the truck. The truck would be capable of towing with an empty trailer battery anyway. Burning extra diesel to charge the battery is usually going to be less efficient than just using that diesel to go down the road. The trailer can charge next time you need to slow down or go down a hill and engage the regen braking.