Maybe you got the wrong impression or I'm misunderstanding you. To test my issue I plan to bring the camper battery down, maybe half. Then load up and take a drive with the fridge on DC. If things are working better, the fridge is still cold and the battery is charging up. I always use gas or AC when stationary and don't see being able to change that until I get Litium iron batteries with more capacity, more solar and a new fridge.
The previous harness was not ony 12 gauge wire but it was also ~12 feet longer and we could not keep the fridge on DC while driving without having a low camper battery upon arriving at our destination.