I get this question about the best dual battery set up fairly often so just a few thoughts. This does not apply to two coach batteries in larger campers and RV's. For these I advise the largest true deep cycle batteries you can fit and unless you have plenty of room and weight in not a concern there is no need for dual six volt batteries. I am writing a more detailed post and article in the next issue of OutdoorX4 covering better battery options.
While I don't want to start a war as this is a touchy subject for reasons I do not understand lets define a deep cycle battery. A dual or “marine” battery is not a true deep cycle battery no matter what sticker is slapped on them. There, I said it. These batteries attempt to fill the gap between a starting battery with high cranking amps and the deep cycle but they are just not “great” at either one. This is not a jab on any specific battery, there are well made dual batteries out there but this really should be a task specific decision in my humble opinion. While starting batteries have many thin lead plates to extract more instant power to crank a motor, the deep cycle battery has very thick plates with less surface area for slow charge and long deep discharge over days to run equipment.
I understand the argument in a double battery set up under the hood with a battery separator/isolator provides insurance if you get a dead starting battery in the sometimes desolate but beautiful places we go. For this small insurance you lose the benefits of a great starting battery and true deep cycle performance of a second battery that needs to run a fridge, laptop, lights, and all of the other gadgets we see at Expo. In higher heat those fridges start pulling much more power to keep temperature as many experienced at the NW Overland Rally in Washington last year. While I liked selling more of our portable solar units than expected to compensate, a good deep cycle battery in place of the second dual type would have helped get through the weekend without running far below the voltage batteries should never go and with proper care will last much longer.
There are dozens of palm sized jump starters that will crank 400 amps or more for that extra starting battery insurance, I bring one wherever I go. It should be noted that there is a separator/isolator in this system and I am not talking about larger campers wiring two 12 volt batteries in parallel to double amp hours in which case you would want the same size, type, and age of battery.
As always feel free to call or email me if you have some specific questions about your set up or what you are considering. Always happy to help.