I've slept in my Model S a number of times at -20°C at 1000 m altitude in the Dovre region of Norway. It's not merely possible it is perfectly practical and plenty warm enough while not really using very much range.
(I have lived for a month in a van in Montana/Colorado/Wyoming. Camped in a Forester for months all over. Lived and worked in a van for 30 days in PNW in spring).