I just replaced the house batteries in our Sea Breeze after getting 8 years out of them. It had to 6v in it. Just curious, why do some people run two 6 volts instead of parallel 12v?
Mostly because 2 large six volt batteries in series gives you more amp-hours than the 12 V batteries do. For example Our 6V Interstates have a rating of 220 AH (amp/hour) each where a 12V might give 80-90 AH each for a total of only 160-180 AH (example). Plus the deep cycle battery handles deep discharge and recharge better.