Batteries in parallel is not a good idea because they need to be well matched and the match does not last as they age.
What you really need to do is to buy 2 6 volt batteries and put them in series. matching is no longer an issue. Suggest you buy real good deep cycle sealed batteries. It will cost you but will last for years.
Manufactures don't do this because 12 volt batteries are cheaper and easier for most owner to deal with.
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