Ideas to extend stay when dry camping/no generator

BFL, intended use is a dry-camp with no generator and 1 or 2 batteries - though it sounds like there will be 2, eventually. As long as it stays above freezing in the night, I personally would not suffer without any batteries bigger than AA. But, since they want to have at least some comfort and plan to carry 2 batteries, it would not be smart to lose too much on charging one battery from another. Though, I would rather lose 25% of charge than carry 60 lbs back and forth...