I just turn off the 12volt master switch and clamp a trickle charger directly to the battery. We get down to -30 around here and never had a problem doing that.
My own experience does not find that rv batteries lose there capacity after a few years. Keep them charged with the correct type charger for the battery involved (very important, different battery type, different charger may be required), and you should get many years of service.
I do the same with the deep cycle batteries for the trolling motor in my boat. They always give me many years of very good service.
BTW, DW and I just got back from a month in your lovely state. We started near Bonners Ferry and zig zagged our way throughout the entire state.