I believe that wiring 12 volt batteries together is not well understood by most installers. It is important to “do it right”. If there are only two 12 volt batteries involved then method #2 or #3 from this page will work:
If there are an *odd* number of batteries then only method #3 will work correctly.
If there are an even number of batteries then method #3 or method #4 will work.
If the batteries are six volt it somewhat harder to “get it wrong”, as there can only be an even number of batteries, and most folks don't get more than four–which makes method #2 correct–and method #1 wrong. The installer is likely to “get it right” when there are only two choices.
So far I've not seen correct wiring diagrams on any of the battery makers pages–so no wonder the installers are unaware of the problem. Most of the diagrams I've found are method #1 which is blatantly incorrect and will lead to early failure of the first battery in the “chain”. Sometimes method #2 is illustrated–which if there are three or more batteries will result in early failure of "end" batteries.
I believe that errors in wiring have helped to create the myth that "six volt batteries are better".