The ability of liquid propane to vaporize (boil) to a gas is dictated by the wetted area of your tank. In simple terms that means the square inches of steel touching the liquid is the determining factor.
This is reduced even more as the liquid level in a tank drops since there is less liquid touching the metal surface thereby reducing the vaporization rate. Ambient temperature is the second major factor to consider since the tanks ability to transfer heat to the liquid is reduced by the temperature also.
Part of the OP's problem nay very well be from flow restrictions in his supply system from the 20# cylinder through the hose and added fittings, as well as ambient temperatures being lower at this higher elevation. A complete series of factors that when combined create the problem he is experiencing.
In cooler weather a 20# cylinder will struggle to keep up with a significant BTU demand which a furnace would be. Add in the other possible flow restrictions and the results are evident.
The newer style OPD valves are not required in cylinders over 40# capicity and would provide full flow from cylinders of any size larger than that.
I would do one of several things to cure the problem. Connect two 20# cylinders together, similar to a trailer setup without the regulator, or go to a single cylinder of a larger size. Absolute minimum of 30# and as large as 100#. Since the problem didn't seem to be there when he was at a lower elevation, and I'm guessing a somewhat higher ambient temperature, I believe the problem is not as much flow restriction as it is adequate size of the supply.
