Someone mentioned 6VDC golf Batteries in series
DO NOT USE 6 VDC BATTERIES
Think about it...
if one fails... what do you do with the other... what does one good 6VDC battery do for you?
The OP just got caught with no battery power in his truck camper to run his jacks (or anything else)
That is exactally what happens when one of the 6 VDC golf batteries fail.
Now... think about where TC people go to boondock... and how often do others really know where you are. Think about an emergency caused by illness or an accident... and you are down to one 6VDC battery.
When you are found you might be on the nightly news... as the unprepared dummy that got stranded along with his family... had to be rescued.
With two 12VDC batteries in parallel... one fails... you'll be OK!
That makes you look like a genius doesn't it, at least to your family.