Lots of people use Little Buddy easy to get online and althought it is rated safe for indoor you need to have a 1 inch window roll down for air. It will shut off for low oxygen or if it tips over. It runs on the little 1 LB propane bottles. I have one and I think it will heat up a truck camper...
https://www.youtube.com/channel/UCL_BZpt0J9Kqwy6YPWr30ow is Mav a youtuber in a F-150 with a camper shell. Almost all his videos are in the snow and you might get some ideas about staying warm.
