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.
One does not need heat to sleep in a car (slept dozens of nights at ski resorts). This does not mean that you can heat up a van/rv with electricity.
(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).
(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).