This gets debated a lot in the boat forums, too! Good, fully charged, batteries don't freeze, even in -30 below weather. The possibility of self discharging gets less and less, the colder the climate. Rather than take a chance that a runaway charger will boil the batteries dry (or that I forget to check the water levels), I just take off all negative leads, and let them sit out in the Minnesota winter until spring. Only takes an hour or two on shore power to get them back to 100%. Those of you where it doesn't get very cold (around freezing, sometimes) may want to think about removal and inside storage/charging. But for us in Chicago and North climates, the cold is a pretty good preservative!
2012 Ford F-150 Eco, Max/Max
Jayco 26.5 RLS, Reese Sidewinder