The reason GC batteries are so great, is the cost factor. Quite simply, after all these years we still don't have a consumer product offering that delivers DC power to an RV cheaper. The price far outweighs the technical compromise of using 6V GC batteries for 12V.
Also, the weight of 1 12V unit with the same cell makeup of the 6V GC would be double and most likely a battery swap would require more than one person.
This is one of those discussions where there are a lot of opinions and one should enter cautiously knowing that no matter what is posted it will be contested.
That said:
There are 3 cells in a 6 volt battery and 6 cells in a 12 volt battery. The outside dimensions are roughly the same. Therefore cell size is much larger in the 6 volt battery allowing room for thicker plates. Thicker plates in a battery provides for longer battery life as long as the battery is properly cared for.
Safe travels. JD
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JD & Kathy and our pom "Mac"
2016 Winnebago Sightseer33C built on a 2016 F-53 Chassis