I had the single group 29 battery with solar, I'd recommend getting solar and you will not have to worry about plugging in. The inverter option however will draw a lot of battery, if you do get it, then go with the dual 6 volts. Here is a link explaining 12 volts The 12volt Side of Life (Part 1)
The 12v pads also draw a lot from your batteries and can only be used while you are plugged in, but if you are plugged in, then you will have electric heat plus/or your furnace. I did not get the pads on my other Escapes.