I use a Mr. Buddy occasionally when boondocking. 12 volt
heating sources take a LOT of power to run. The fan motors in forced air furnaces will eat a pair of group 27 batteries over night, if the temps are cold.
The space required between the heater and combustibles is more a function of how much heat the burner is producing than anything else.
Catalytic heaters make radiant heat. It heats objects, not the air. A few of feet of separation is probably ok. Test it one day, fire the heater up and monitor your bedding. Position the heater so the bedding is warm, but not hot to the touch.
As for venting, the main issue with catalytic heaters is the depletion of oxygen, not the production of carbon monoxide. A couple of cracked windows will be safe. Most RVs leak enough air to be safe. If they didn’t, they would not be so hard to heat. Convection losses noted.