If 110v is available, why don't you just plug your shore power cord into a 110V outlet (using the proper adaptor, of course). It'll keep the batteries charged and I can't imagine you'd have a problem with them freezing. You'll still need to check the water levels monthly but I think I'd rather do that than remove them. When my Baystar is plugged into 20 amp power it only draws 2 amps.
edit: I just did a couple of quick online searches and one fellow reports that a fully charged battery will not freeze until approximately minus 76 fahrenheit. But then you can't believe everything you read on the net.
Marshall Valentino, Beattie, TX
2019 Baystar 3124
2016 Chevy Colorado Toad, Blue Ox, RVi brake