I used the mr buddy in a fold down camper in some very cold weather and it worked fine , leave a vent open for fresh air. they do use a lot of fuel so you might want to hook it up to a 20lb. tank. the small bottles only last 4 to 5 hours.
i have a mr. buddy i use in my van. called the company and they say you need 10 square inches of vent, which is only about a 3" square. i crack both front windows about an inch and have a carbon monoxide detector. they have a low ox. shut off also.
I had a slide in pop up truck camper and used a Coleman Black Cat heater. Worked good. IMHO I think it is a little safer than the MR Heater which I also have.
I had insulation for the fabric part of the sides. This really helped alot.