electricity follows the path of least resistance. so when your truck is off the easiest less resistance path is from the camper batteries. when your truck is running the alternator is putting out 14 or more volts at whatever current it can produce at the rpm of your alternator pushing it to the camper. that is why it works great with the truck running. can you make it work better with out the truck running? you could lower the resistance by increasing the size of the 6ga wire. or others have talked about using buckboost dc to dc converters.
