We live in the same temps as you stated and we have always just left our rig connected to shore power 24/7 no matter the time of year as it is stored in our side yard. We have 4 year old 6 volt Interstate batteries from Samsclub and they are holding up well being charged by our Iota DLS converter.
Trickle charging, activating 12 volt kill switch with occasional charging, or just removing them altogether will work as well. I don't do anything with the batteries in our motorized toys or work equipment over the winter, just charge them with the BD 40 amp charger before and after the winter season comes and goes.