Could you insulate the batteries to reduce the heating demand? Instead of heating the entire cabin just heat the battery compartment, and only to the minimum necessary to keep them from being damaged (plus a margin for safety).
Maybe even just toss them in an old discarded deep freeze? You could even have one battery in there that is disconnected from the rest of the system so it can be used to jump start the system if it ever gets fully discharged.
Maybe even just toss them in an old discarded deep freeze? You could even have one battery in there that is disconnected from the rest of the system so it can be used to jump start the system if it ever gets fully discharged.