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 between 75 and 78* overnight using the 600 watt setting and the dial on 2. I cracked open a floor vent for some fresh air through the night. No fans for these heaters. I'd shut it off around 5 AM because it was to warm inside to sleep.
The Buddy heater was used in January for 20 days in 20* temps. Heated ok but would probably work best in warmer ambient Temps and water vapor meant I needed to keep a fan going and a window open a 1/2".
An Olympic cat heater would probable work better as they are more efficient at burning all the propane. Again, I'm wintering in the NE so keeping all the heat in is a priority.