Hot air rises.
So the lower the better.
We place our electric ceramic cube heater on the floor in front of the closet. (I know, you didn't ask about the electric heater, but as I said, I really recommend having electricity for January campouts!)
It's a different story with the little propane
The Black Cat doesn't have a tip-over protection built in, like the Mr. Heater Buddy ... so if your dog, kid or husband on a late night potty break might tip it over (because it would keep burning in a tipped over position!), you want to keep the heater in the open sink.
The Black Cat is a true catalytic heater ... and burns a lower temperature than the Buddy heater.
Coleman's two-foot recomendation is to prevent items from melting from contact with the head ... not materials starting fire.
The Mr. Heater Buddy burns hotter, and produces more heat (but as you know, doesn't burn reliably at altitude)(which is why we own one of each, now!).
I also always lite either one outside the trailer ... due to an very realistic fear that a broken seal or small propane
leak might cause an explosion or fire! Much easier to drop it and run away outside than it would be inside your trailer!
I've always been worried about our dog's tail swishing into it, anywhere on the floor of the trailer, so again, we usually keep the Mr. Buddy in the open sink.
If you don't have a dog, or a husband who's prone to tipping things over (like me), you could put in on the bathroom floor.
Remember to ventilate your trailer, by keeping the bathroom window cracked and the top vent on the trailer open, no matter which heater you use ... or where you use it!