My 2003 8.1 Burb averages 9 - 9.5mpg towing my 7200# TT at 65mph. With a 36gal tank, I start looking for a station at about 4 hours(1/4 tank) or 250miles. That's more than most people want to travel without a pit stop.
As far as driving in the mountains at higher elevations, yes there is some noticeable loss of horsepower. But it has plenty to spare to begin with, with diesel like torque. You won't find another vehicle with 4x4 other than the Excursion that will hold up the tongue weight of an 8000# trailer and carry 6 to 8 passengers.
I'll not be giving up my Burb anytime soon!
