I tow my 8000 k TT with T/H. I have never had to lock out the overdrive, not even going up the mountains to Vegas on the I-15 or from Bakersfield to Tehachapi. The T/H engaged will help slow you down going downhill.
Truck just wants to grunt and pull...it makes it's tail gate wag!!!

 

I am getting ready to tow a 5k boat up and down some good mountains with 10% grades. Is it better to keep the revs up for lower EGT's?

 

If you're going up a long 10% grade, and your speed starts to drop, and your engine starts to lug, lock it out of overdrive. If you're going down a long really steep hill, select lower gears to avoid burning out your brakes. On a really long steep grade you are better off in low gears going more slowly- even 1'st if necessary. Don't ride your brakes.

 

I just want to tell everyone that I appreciate all of your comments and opinions as Friday I am hooking up and going. Just wanted to say thanks once again.