47,000 miles on 2012 F350 4x4 SRW CC (3.55 gear). Anywhere between 14 and 18mpg non-towing. My overall average towing FW (just under 16,000 pounds) is 11mpg. I have towed in 30mph winds on hilly terrain and only gotten 7mpg, and then I have towed with a tail wind on flat ground and hit close to 14.
Reference the small tank on the 6 3/4' beds. When towing I will generally run about 20 gallons out before fueling. Since I only run about 60-62mph that takes me about 220 miles, or 3.5 hours.
That still leaves me well over 50 miles until empty, plus I will carry 2 five gallon cans of diesel in my tool box just in case. I am picky where I purchase my diesel so I think I ended up using my spare cans twice last year.
I am very close on my pin and other weight limits. If I added an auxiliary fuel tank (300 - 500 pounds) I would just be hauling needless extra weight IMHO. If I was in a hurry, or could actually sit in the seat and drive without a break for 8 or 10 hours then I possibly would consider hauling 50 - 80 gallons of diesel down the road.