I often say "There are no six volt batteries in RV's" People get upset cause they have 2,4,6,8 of them (heck I have an even number of them too) but the fact is they are only six volt batteries when they are OUTSIDE the RV.
once installed, you put each pair (Why they are always in even numbers) In series and like magic.. (Or assorted laws of electricity) you have a 12 volt battery,, To be specific a 4D.
For winter storage.. Do the same thing... Put 'em in series and treat them like a 4D.. One big 12 volt battery.. For that's what they become when you add that jumper wire.