>any Deep Cycle battery will give longer life than a regular auto starter
>battery... I'm sure golf cart batteries are Deep Cycle... and Marine
>Batteries are too... but 2 6v batteries that add up to the same Amp Hours
>as a 12 volt battery is rated for should give the same number of hours of
>performance with the same load... It all comes down to Deep Cycle and Amp
>Hour rating... nothing mysterious...

Well, just to split hairs, "Marine" batteries are not deep cycle
batteries. They tend to fall in between a car battery, and a true deep
cycle one. Of course its a lot of semantics, but manufacturers will try
and get away with what they can. Buy a  true, deep cycle battery and be
safe. Decent sized deep cycle batteries tend to come in 6V packages (size
being amp-hours) so one usually ends of buying a pair of them, especially
when looking for a lot of amp-hours.

Its a roaring battle in a lot of other groups, but just be careful that
you get the right battery for the application.

John