it is correct to say, "Increase the wire to #4, but how to do it? Where is the route? Eg, we have the trailer door in the middle and the converter/ power centre is on one side of the door and the front where the batteries are is on the other side of the door. No cabinets where the door is to hide any wires.
Can't be done! Only way is to get the converter and batteries on the same side of the door. You can't use the wall cavities either--total zoo.
Easy for us, just stick the "deck mount" converter up where the batteries are and leave the original wiring alone. No fuss, no muss.