I tow up some steep hills but it's nothing like the Rockies. The steepest grade I climb is the mass pike heading to lake George or in the white mountains of New Hampshire
I too know a friend that tows his 14,000# fifthwheel close to 10,000 miles a year between Massachusetts, Ohio, Pennsylvania, and Illinois and he opted for the 6.2 liter, 4.30 gears and is happy with them. I used his truck to tow my Montana home last week and it did just fine. It downshifted to 4th once while climbing a bridge but do to traffic I couldn't get a running start at the bridge.