For a small heater, I use my Big Buddy heater.
It will heat close to 400 sq ft, and has the Low Oxygen Cutoff.
It's desigened for tents, and works great. The Big Buddy also has a small built in fan, that runs on 4D cell batteries.
It's a bit big for my RTT, but I use it to warm it up inside, so I can get dressed:coffeedrink:
I use a heavy sleeping bag, so I don't need to have heat all night in the RTT.
While using a friends pop up tent camper, I did run it all night, and it worked great.