F-250 and a 5th wheel questions.

I have a 2005 F-250 crew cab shortbed with the 5.4L (300HP and 365 ft/lb) and 3.73 gearing. It has everything needed to tow. (Slider hitch, trailer brake controller, tranny cooler, etc.)

I know the truck will pull my new trailer (2005 Springdale 249 bunkhouse 5th wheel). The trailer weighs around 7000 - 7500 pounds loaded. My wife and I are not new to towing or mountain driving, FYI.

However, I am curious if anyone has traveled the Alaskan Highway with a similar setup. I am considering a different truck (another F-250 or F-350) with a larger engine and/or deeper gearing. I dont want a diesel since the trailer will only get towed about once per month and the diesel is not suitable for short trips and sitting in traffic like we will be doing with it. (See my post "Anchorage, AK to Washington D.C." for background info in the "Destinations" section). My biggest fears are gas mileage and time. I am expecting to get about 8mpg, but I think I am being to optimistic. Also, I dont mind going slow uphill, but I dont want to be TOO slow. Less time pulling the passes means more time playing at the destinations each day, lol.

Anyone have any experience or feedback?