I've been using a Buddy in my small ice fishing shacks for years and always open a window/flap. I recall reading a test that someone did in a 55 gallon drum with some sensors, attempting to verify that their Buddy's oxygen depletion sensor works as advertised. The unit shutdown within 90 seconds at the appropriate level (or lack of) oxygen in the 55 gal drum. Our campers are much larger than a 55 gallon drum, but it still gives me pause. In a canvas pup I suspect it would be fine due to it not being exactly air-tight. I always considered using one in my QS10 tent camper but never had the need. However, I'm not sure I'd use it in my hard-walled Aliner which is a tad tighter - although I know some folks do use them in A-frames.