Heating the batteries and passenger compartment is quite expensive energy-wise, you certainly have to calculate that in when determining an appropriate battery size for such an application.
Definitely something to consider. I wonder if there's some value to a secondary/supplemental seasonal battery that can be added for part of the year to handle the extra load.
Would come down to whether the installation/removal (to lower the "dry" weight for the warm seasons) would be worth the energy savings of not hauling around the supplemental battery weight.
Other options might be to use some kind of thermal mass for heating, perhaps a large heated mass or fluid tank? Not sure if there's any benefit there though beyond being able to swap out the fluid tank between runs faster than charging a battery.
You don't need to heat the entire passenger compartment unless folks are in there, though. You just heat the bits that could benefit. This is the same sort of thinking behind the engine block heaters (for combustion engines) folks use when it is cold and a similar theory to trucks that heat their diesel tanks to keep the diesel from turning to gel, though I don't know if that's an issue anymore with modern diesel blends.
Would come down to whether the installation/removal (to lower the "dry" weight for the warm seasons) would be worth the energy savings of not hauling around the supplemental battery weight.
Other options might be to use some kind of thermal mass for heating, perhaps a large heated mass or fluid tank? Not sure if there's any benefit there though beyond being able to swap out the fluid tank between runs faster than charging a battery.