Resale value is not that important to me...I tend to keep vehicles forever.
I currently drive a Ford Ranger that is 22 years old. (Hard to believe I was 34 years old when I drove that off the lot!)
My commuter Corolla is 16 years old and just went over 270,000 miles. Our "newest" vehicle is about to turn seven.
This is potentially the last truck I will buy. If I keep it as long as my current truck, I will be close to 80.
So, yeah...the additional cost of a diesel over a long time is not really a factor.
After considering everything, a diesel 350 sounds like a good choice. I would still like to avoid the dually, though.
According to Ford's numbers, I would pretty much have to rule out the F250 diesel as it's cargo capacity with a long bed is 200 lbs below the hitch weight of this fifth-wheel, and 800 lbs below the gas engine capacity (4x4 numbers).