See, this is where the personal choice comes in. There's no "right" location for them. I'm putting them outside in a compartment. Others put them inside. Put them wherever it works for you. You just have to deal with the consequences (applies to all choices to be made).
Note that superdave's freezing comment was about a particular circumstance. Batteries don't freeze in normal use - otherwise they wouldn't be very useful in a car or bus parked overnight in winter, would they?
A PV system can draw them down enough to create some risk, though. But you could also resolve that with a compartment heater - either one of the mats used to keep water tanks/lines from freezing, or something like a "Goldenrod", which they use in boat closets over-winter to dehumidify them. It doesn't need to keep the battery at 70...
SO much to think about just in this one point. Wiring, service access, gas venting, space usage, weight/balance, etc., etc.