That's objectively horrible yeah. I guess if it's deep winter and you have any reason to think you might have to camp out in sub-freezing temps, keep some supplies in the car. Mylar "space blankets" and chemical hand warmers are very compact and easy to just keep in the car. Maybe some a normal blanket and a packed lunch is the only extra you'd need to remember to bring.
I'm sure someone can or already has measured it, but I figure the car "idling" with the heat on would use maybe 2kw so depending on the SoC you could survive but might need to recharge to finish your trip... don't count on getting full battery capacity at those temperatures either.
Using the heated seat with a decent blanket would be a great way to save battery.