As enblethen mentions, the fact that they're 6V batteries matters not a bit here; a pair of 6V batteries in series is a 12V battery, albeit one physically separated into two sections.
I do it by plugging in my motorhome and letting the (multistage) converter do its job. If that's not an option and the storage is not indoors in the dark, a modest solar setup is a great option. For short periods of time or storage in cold (sub-freezing) conditions, disconnecting fully charged batteries is reasonable. Failing those, the best choice is to remove the batteries to some place where you can keep them charged...though that's hardly convenient.
