The 6V versus 12V battery discussion always draws debate, and this one inspired me to step out of my cubicle here at work at Motorola Solutions (the 2-way radio communications company) and ask an electrical engineer. He has a masters degree, focus with power. He also happens to be my 29 year old son. He quickly reviewed our motor home scenerio and concluded this.GIVEN IDENTICAL AMPERAGE BATTERY RATINGS FOR BOTH 6V & 12V BATTERIES, two 12V batteries will supply twice the power of two 6V. I did a quick search on deep cycle batteries and found the typical 12V to be around 100Ah, and 6V to be around 200Ah. If this is the comparison, then they hold equal amounts of energy reserve power.IN THE EVENT OF ONE BATTERY FAILING....12V - The remaining good 12V will get damaged if the condition continues for an extended period of time.6V - The remaining good 6V will not be harmedIf one battery dies during a vacation and you catch it in time, you can isolate the two 12V batteries and continue on your trip with only one 12V battery. In the case with 6V batteries, you will simply see a voltage drop which you can do nothing about until you replace the bad battery. Given it is recommended to replace batteries in pairs, the 6V benefit seems to hold no value. I also wonder how hard it is to find a pair of new 6V batteries in the middle of No Where USA.CHARGING 12V and 6V BATTERIES THAT ARE IN PERFECT WORKING ORDER12V batteries will charge evenly, 6V batteries will not. If you vacation with limited charge times as we do, this could be an issue with 6V batteries, potentially unable to provide a full 12V if the charge is not allowed to complete.My son is not familiar with lead acid battery technology, so there could be influencial factors that sway benefits to one versus the other.Zorba,Your isolation diodes for 12V sound like a good idea, though few people would apply it.Your support for two 6V in series has a strong following so I am not really convinced one way or the other.Just thought I'd put this into the discussion.