We're towing with a 2003 F350...6.0 Diesel. Luckily we have one of the good ones!
We've been driving all around the Appalachians/Blue Ridge mtns for the past 5 weeks with grades up to 13%. Some of the towing has been on the back roads, not all interstate driving. We're happy with the way our truck has handled all of this. Even the steepest grades the rpms never hit over 3000. Towing mileage has been between 11 to 12 mpg, on the worst steep winding back roads it went to down to 9mpg.
You want any further technical info you wont get it from me, Gary will have to chime in there.