My first question is: do you do any Winter camping?
If the answer is no, then I would recommend removing the batteries when you winterize your camper and keep them in a place that isn't subject to freezing. I would recommend this for either Lead Acid, AGM, or Lithium. Lead Acid or AGM won't freeze if they are fully charged, if they are not fully charged, they will freeze up and break the case and leak acid.
If you switch to Lithium, you can mount them inside the coach (under the dinette or at the bottom of a closet are popular choices) where your furnace will keep them above freezing (Lithium batteries do not need venting). The issue with Lithium batteries is that they can not be charged if the battery temperature is 32 degrees F (0-C) or below. They can still be discharged below 32 degrees, just not charged. As far as Winter storage goes, check the manufacturer's specs for the lowest storage temperature (personally, I would remove them for Winter storage).
Many Lithium batteries have a Cold Temperature shut-off that will keep them from charging if the internal temperature is below 32 degrees, you definitely want to ensure that you buy a battery with this feature.
Bottom line: I would definitely recommend switching to Lithium (LiFePo4) batteries and removing them during Winter storage.