I use a MR BUDDY heater for my backup heat source but have to open a couple of vents to insure all of the oxygen is not used up over night and poison gases get vented out. Seems like a waste of heat if you have to open vents to use it safely.
The only way I would do this on a permanent basis would be to install a WAVE/CAT propane heater that has outside venting capability...
Here is a couple of photos from google showing a CAT Propane heater installed on a cabinet door that has external vent capability...
I recently read a Truck Camper install on here built a wall across the bottom of the passage way to the truck rear window to house a WAVE Propane Heater. Looked pretty neat and was out of the way... After thinking about it you probably would burn you legs getting out of bed in the mornings haha...
Roy Ken