IAdie,
What differential axle ratio does your truck have? 3. 54:1?
I ask because my truck used to have a 3. 54:1 axle ratio, and when I pulled my travel trailer (which weighs about 8500 pounds loaded) I used to see high EGT's when trying to climb anything but the slightest hills in 4th gear (OD). The engine was essentially lugging under this condition and the EGT's would climb without any appreciable increase in power. I was forced to downshift and spend a lot of time driving in 3rd gear in hilly terrain and even when towing into a stiff headwind. I should add that I had a 4" exhaust system... that won't help.
Then I had the axle ratio changed from the original 3. 54:1 to the factory optional 4. 10:1, and all I can say is WOW! It's like driving a completely different truck. Now I can leave the transmission in 4th gear on all but the steepest mountain grades and still keep the EGT's within limits. In short, the axle ratio change made towing a pleasure.
Of course anything you do to increase available engine power is going to help, but you have to be careful not to overload your stock transmission... it's only designed to handle a ceratin amount of engine horsepower. On the other hand an axle ratio change avoids that problem.
A few weeks ago I finished a trip towing my trailer from the Seattle area to Death Valley, CA and back... with many steep mountain passes in between. I averaged about 11. 5 mpg for the trip.
Also, if you haven't done so already, invest in a good exhaust brake and a transmission lockup controller... once you tow with them in mountainous terrain I guarantee you'll never go without them again!
I hope that helps,
John L.