Propex US distributor responding :-)

No problems at altitude if the regulator is set properly and directions sent
with the heater are followed. There are quite a few that have gone to 10-11K
feet with no problems.
The HS2211 was designed to be used outside. If exposed to a lot of nasties,
just make a protective cover of sort. The case is galvanized steel but you
could add a protective coating over that if you wish.
If you have installed, or are thinking of installing, one of the new
electric fridges, those leave enough space for the Propex HS2000 to fit
neatly beneath. Other mounting options include Rocky Mountain Westy's little
storage box behind the passenger seat, and in the small cubby as mentioned.
Under the rear seat you only lose a spot 4" tall by 10x20. Feel free to
contact me directly for any other related questions about these heaters.

Karl
www.propexusa.com