I've used our Mr Heater Buddy heater many times with no issues. Low oxygen shut off is built in, keep the place vented, etc. The heater can be placed right on the cook top where it should be out of the way. Use the heater to get the place toasty before going to bed, then shut it off.
We just got back from camping where the nights were below freezing. Our little trailer never dropped below 50 with just the two of us in it and furnace off. 50 is not that cold, especially if you're under a blanket. It's perfect sleeping temp. Your camper will probably be totally comfy inside if it's 50 outside.
A few years ago, when my wife was undergoing chemo, she would freeze and just couldn't get warm, especially during the treatment. I bought a 12v throw blanket that draws 3 - 4 amps I think. It would last a couple hours on the little battery pack I added to it. Just a thought.