Bob,
The main thing you're getting with the 6V batteries is longer life in deep-cycle use. You might also think about a minimal solar setup to help the batteries out--it wouldn't take much to keep up with what the refrigerator needs, at least in the summer time.
It also matters (quite a bit with lead acid) how fast you're drawing power as to what the battery can deliver. In your link, note the different rate columns--you were actually down more in the 6-hour column (187Ah), not the 20-hour rate. Good luck finding a similar chart for those group 24 12V batteries--probably doesn't really exist because the numbers likely fall off a lot faster.
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