Jon,
A 20lb propane tank has approximately 439,500 btu's available (at 60 degrees F). The Propex 2800 is rated at 9,700 BTU max. At max output you should get approximately 45 hours out of a tank...in theory.
On the elk hunting trip I used a Mr. Heater Big Buddy 18,000btu heater for my annex/extra room. It did OK until we got 6" of snow and temps dropped into the low 20's/upper teens at night. I also used it in the main tent area of my KK to help assist the Webasto 2000...since, at the elevation we were at (7,500'), the Webasto just wasn't able to perform satisfactorily due to the combined low temps, breezy conditions, snow and 7,500' altitude.
I have a difficult time believing the Big Buddy puts out an honest 18,000btus per their rating. And bring along plenty of extra D cells as it tends to chew right threw them...ditto for 1lb propane canisters unless you opt for the bulk tank conversion (if you do go this route be sure to run an inline propane filter from the bulk tank or you are guaranteed to have issues with the Buddy type heaters...).
I like the concept of the Propex 2800...but I'm leary it could, at its 9,700btu rated max output, keep the air volume in the main area of our trailer tents warm enough at sub-freezing temps, windy conditions and higher elevations...given the absence of wall/ceiling insulation.
Have you contacted Ken Romer? I believe he has one of the Propex heaters in his KK. Not sure of the highest elevation and how cold he's used it but it would be worth an e-mail or phone call...
Maybe the Atwood, that was mentioned by Christian P on page 2 of this thread, would be worthy of consideration. The only negative, besides the larger footprint, might be the electrical draw...but maybe worth checking out.