Need heavier springs to carry the extra battery weight
As soon as the 4 batteries were placed in the spare tire box, and the suspension sat right on the overload stops, it was pretty obvious that beefing up the suspension was going to be necessary. After exploring the purchase of heavier springs, I settled on the use of helper air springs.
Air system components
The advantage of air springs was the ability to adjust the suspension in real time.
If I go to lighter batteries in the future, only a twist of the air regulator will be needed to properly get things back to normal suspension travel.
The air springs require an air supply, so I made one from a 12V compressor, a pressure switch, and used the spare tire as both a cover and an air reservoir.