I'm still nervous about these. Manufacturer says dont use them in doors. Terrifying and puzzling from the start. On the other hand I've watched YouTube videos of people testing them with testing devices in trailers and tents and never setting of the alarms. So I don't know. I've decided to go the diesel heater route and stop worrying about it.
In theory, a propane fueled flame will produce CO, but the amounts we're talking about would really only come into play in a small, sealed environment. Their website even lists them as safe for indoor use, though I believe they specify in a well ventilated area. I've used them indoors during an extended power outage, and in tents several times. I carry a carbon monoxide detector with me just to be safe, but so far never a peep. And the unit has a low oxygen sensor that will shut the unit down if the saturation levels get too high.
All that said, I'll probably end up getting a diesel heater eventually especially to avoid the condensation fun, but as I don't do that much cold weather camping, it's a lower priority for now.
The biggest problem I have the the Mr. Heater is if you connect it to a larger tank (at least in the case of my 5lb tank) there seems to be a very specific way you have to open the valve. If you just hook up the hose and crank it open, it never lights (some manner of vapor lock maybe?). You have to ease it open, but not too slowly. Somewhat annoying trying to get it right.