You should replace your group 27 with a larger battery if you have room. If it's a wet cell battery mounted in a sealed box thats vented outside, I would measure the interior dimensions of the box to see how big you can go. You might be able to fit a group 31 in the same box.
When I replaced the batteries in my SMB (RB30 layout), I put in two group 31 AGM batteries. With AGM batteries, you don't have to stress about the off gassing into the cabin. If you decide to run two batteries, you would just need some extra wiring to connect the two batteries. I went to a local stereo shop and had them make me a couple custom length wires.
I think Boywonder found this guy in SD that sells remanufactured or rebranded batteries. He bought some for his van and if you've seen his build, you would see that he doesn't cut corners. Heres the link to the battery guy...
http://deepcyclebatterystore.com/12-volt-rv-battery/