Hi Vermonter.
I lived in Vermont for about 12 years, I recall -40F nights that usually happened every year just after Xmas. I don't recommend any kind of battery for going through that.
I use a large AGM battery bank (24v 1400AH) but the lowest my set has seen is -20F, and probably not that much as it's in a barn, unheated though it may be. As you mention, the key is to not draw them down too much (I never let it get below 80% of full charge). It's even more crucial when they are cold, as the capacity decreases with cold.
A possible source for these batteries is the telephone company, which uses large 48v banks to back up the phone system and cycles them every few years. They often have never been cycled, having sat at float for years. It's worth getting to know someone there as you might be able to pick up a lot of battery for not too much $.
