The two Interstate 12 volt deep cycle batteries that came with our motorhome lasted less than four years. I replaced them with two Trojan T-105 6 volt deep cycle batteries in June 2007. These batteries are still going strong after eight years. I don't worry about failure of one 6 volt battery, as deep cycle golf cart batteries are widely available.
I don't know the difference in amp-hours, but the Trojans definitely can run the furnace much longer without getting to 50% discharge compared to past experience with the 12 volt Interstate batteries.
The Trojans are larger (taller) than my old 12 volt batteries and are heavier. I figure that more lead in a battery means more amp-hours.