You need to keep in mind that any given voltage applied to lead acid battery terminals has less of an affect in charging them or maintaining them or boiling their water away as the temperature gets colder.
This means that the good old 13.6 to 12.8 volts from a single stage converter, left on batteries continuously during cold weather, will not have the detrimental effect that it could have during warm weather.
During hot weather I disconnect my 13.8V converter from the house batteries while the RV is in storage. During the winter in storage, I leave the fixed voltage converter hooked up to the RV batteries 24/7.
My batteries are over 8 years old and still going strong using this approach. They're AGM batteries so any water boiled away cannot be replaced ... so damage is irreversible.
