My 87 F250 HD (1 ton rear, 3.55), C6, NA 6.9 ... in high wind when pulling a 27ft aluminium frame old travel trailer (pretty short vs modern ones), I could barely stay at 50mph in high wind conditions on flat roads with the pedal to the floor. Turbo would have helped a lot. I never towed it in the mountains thank goodness. That would have been miserable. The truck without a trailer in the mountains is slow as all get-out as it was.
Now I'm driving a brick of a motorhome with 6.9 ATS 085, C6, 410 Dana 70 dually, and there is a definite difference. It's taller than the trailer was by a good several feet so it's a bigger wind break from all sides. There's no way I would have wanted this thing if it didn't have turbo. I keep my foot out of it mostly for better MPG and since it's unlikely to have head studs from when it was installed back in 1985.
If you can afford to add turbo, do so.