2002 F350 4WD single-rear-wheel 7.3 liter diesel, Crew Cab, longbed, auto trans (tried to find a 6-speed when shopping for used in 2004, but they're scarcer than hen's teeth here in the East). Now with +239,000 miles. Have yet to mount the Palomino pop-up camper purchased well-used in late 2012. Have had a heavy fiberglass shell + way too much in the way of tools and gear aboard since 2010. Standard 38 gallon tank. No "chip" or other mods, save for a K&N air filter. I run Rotalla T6 full synthetic oil and Mobil 1 full synthetic ATF. A simple oil and filter change thus runs about $100 even with DIY, and the full ATF pumpout and refill, which consumes 20 qts of ATF, runs just under $200.
Mostly by keeping my speed down in the 70 mph range, or less, I clocked an honest 17.1 mpg on my last summertime trip out West. That was a 7,017 mile round trip and involved about 400 miles of "bombing around" on trails along Montana's Rock Creek and in the Pioneer Range west of Dillon, MT. Ran the A/C some of the way out and all the way back, and 2 of the 4 days of the return trip were in +100 degree weather in the Midwest and Southeast. On that trip, I had a good 1,000 lbs of shell, camping gear, and fishing gear on board. On one tank I reached 570 miles and on another 607 miles. Each day's travel on those tanks involved a steady and big tailwind and no A/C, and with the aforementioned light foot on the skinny pedal. My onboard gauge in those instances was reading 18.5-19.75 mpg, and the volume purchased at fill-up was under 35 gallons, so I had a needle's width still showing.
On lightly-loaded "bombing around" excursions on halfway decent gravel roads, without a lot of climbing, running 40-45 mph keeps my gauge in the 21-23 mpg range. I've never run it enough in that mode to really test the gauge, but it's pretty much spot-on for open highway calculations.
She's an oldie but goodie, and as long as I can keep her, I intend to.
Foy