Larry, if you go to the DC Electrical section in the RV Doc archives, (scroll down, on the right-hand side), you’ll find a bunch of similar questions; some with cool diagrams. But two, 6-volt batteries will provide almost double the current storage capacity of a single, 12-volt battery. I often recommend using 6-volt batteries in place of TWO, 12 volt batteries. Here’s why; using simple math, two 6-volt batteries wired in series will typically hold more amp-hours than two 12-volt batteries connected in parallel, utilizing the same foot print (taking up the same real estate space). Two Group 27, 12-volt batteries have the capability to store approximately 105 amp-hours. Double that for two batteries wired in parallel; 210 amps approximately. Two Trojan T-105 6-volt batteries wired in series have the capability to hold about 225 amp hours. In series, the voltage doubles, but the amperage remains the same.
Though minimal, the storage capacity leans in the favor of the twin 6-volt batteries. And with the 6-volt batteries you also get true, deep cycle, commercial grade storage. They’re designed to be cycled over and over.
So from a clinical, purely mathematical, point of view, I give the edge to the 6-volt configuration. That said, there are many outstanding 12-volt batteries (think AGM), that can be just as effective, so in reality, it’s probably a wash when compared to two, 12-volt batteries, but you’d certainly gain storage capacity when replacing just a single, 12-volt battery. And if you are prone to dry camp, those extra few amp-hours may just come in handy.