Might I suggest that if you have access to the January issue of Trailer Life magazine that you refer to the towing guide. Basically, the suggestion is that one will want to use a TV rated at 125% of the gross load of the trailer. As I recall, at elevations above 7000', the TV will loose 10% for every 1000' of elevation. At just 10,000' elevation, the TV will have lost 30% power. (I suggest that one place more emphasis on engine torque rather then horse power.) Keep in mind that the engine is on the verge of having an internal hemorrhage because of the thin air.
You may hear an old timer refer to this as the "Rocky Mountain fudge factor" which says that one never exceeds 80% of the TV rated capacity.
Having lived in Denver, I've found this to be a most valuable rule of thumb.
Best of luck in your travels.