I have an Exide Group 27 "dual purpose" battery in my AS, that I got at Tractor Supply for $59. My truck has dual batteries, but they are cranking batteries, not deep cycle or dual purpose. When the trailer is connected to the truck, both batteries are connected together. I have seen no ill effects to either battery with this arrangement. Have gone 2 days without hookups using this arrangement.
2003 GMC 2500HD 4X4 D/A Ext. Cab
Propane Powered Honda EU2000i
Lots of Hot Sauce!
Air # 283