Just my 2¢ on heaters.
I've got a 20 yr old Oil-filled radiant heater. One Switch for 600 watt and a second one for 900 watt, 1500 w if you use both. Plus a dial 0-9 for heat range.
Last winter connected to shore power at 18-25* through the month of February. My box van, well insulated, stayed...