What is the point.

The way I understand it, any flooded lead-acid battery should never be discharged less than 50% without causing damage to the cells (hence shortening the life of the battery). Given the data you provided, 50% discharge of your 12v battery yields approx 57 amp-hours. x 2 is 114 ah total usable capacity. The 6 volt batteries at 50% give 116 ah EACH, so you should have approx twice the usable amp-hours if you were to change to the (2) 6v, which are true deep-cycle ones. The 12v are likely trolling-motor style, meaning they are capable of providing high amperage discharge rates to start things, not just a steady discharge like a true deep cycle. All the other comments on 12v vs 6v are accurate, meaning one dead 6v battery leaves you out of service, etc. But unless you do a lot of extended boondocking without either running the engine driving about, a generator running, or solar panels, then having the extra battery storage capacity is rather meaningless. Hope this helps! ST