OP, glad you found "THE ONE"! Sharp truck... once you get past it dropping gears on grades, like 2nd gear at 4grand running 65mph, you're over the hump ;D
While I've had a threesome of tow rigs over the years, each with their own limitations/irritations ('92 F-250 4x4 7.3NA/5spd = dog on any grade; '93 D300 Cummins/A500 = auto ran hot ALL the time; '99 Sub 5.7/4L80 = 2WD)... I'm about ready to start looking again and the info here has been great.
We generally use my brothers '01 2500HD ECSB 4x4 8.1/Allison (still under 75k) . Over the past 14 years, hauling my 4200# Commando with a variety of trailers (yardsale variety car trailer @ 1200# dry; hydraulic dump trailer @ 4200# dry; my 5ton equipment trailer @ 2700#) the truck consistently gets 8.75MPG... uphill, downhill, tailwind, flat ground, speed limit/10 over, tow-haul, cruise on/cruise off... never a change from 8.75MPG (hand calc'd). He swears he gets 13MPG towing his 19# TT