I have owned my Propex for 10 years and it has always functioned properly at altitude. Though,we have never camped over 10,000 ft. I removed my rear stock heater and installed the Propex in it's place. Very little lost space. At the time the outdoor version was not available. 

      woody 
"keep on pushin"

> Date: Mon, 29 Sep 2014 14:51:35 -0400
> From: tdiguru@WESTYVENTURES.COM
> Subject: Re: Propex + altitude +placement
> To: vanagon@GERRY.VANAGON.COM
> 
> 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