The only issue at all I had pulling my 40' fifth wheel (12,300 lbs unloaded) with my 3/4 ton Ford is that it dropped the back end of the truck enough that it made the steering light at highway speed, which appears to be what Dave was alluding to with the GVWR/payload issue. I took care of that by adding a set of air bags that are adjustable from the console of the truck. I've pulled it as far as 16 hours away from home without any towing problems. The truck drives great with the bags inflated to 60 psi, which levels it out nicely. As for the Chevy diesel and Allison tranny, you won't have any problems at all.
Mike in Tennessee
Former 2 time FR owner
2007 Crossroads Cruiser 30QB Fifth Wheel
2008 F-250 Lariat 4x4 6.4 Powerstroke
Firestone Ride-Rites w/cab control