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If both batteries are at the same voltage all the time, you'll end up deep cycling them both. Can you find room in the engine bay for a marine dual purpose battery?
There's also the junkyard battery option. Buy fresh 51-series batteries for $15 each, and abuse them. You'd have to get at least a year out of each battery for this option to be economical.
VegasDude gave me an idea which seems to be worth thinking about. Run your starter off of one battery, and attach the power distribution box + the alternator to the marine battery in the trunk. You can get away with narrower cable that way. You can grid charge both batteries when you get home. I hope you don't do too many key starts.
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