An 01 F350 should have 3.73 or option 4.30 gears. If it has a tachometer, you could tell by figuring out the speed to rpm comparison. If you can't figure it out, just post the rpm at 55mph and at 65 mph. By posting the two speeds, can better check the calcs. Would also need to know the tire size.
Based on 3.73 gears, empty the truck should get around 12-13 and pulling a trailer about 9. TC may do a tad bit better, but usually about the same due to wind load. The 4.30 gears would hurt empty mpg more than loaded.
The V10 was a good engine except for the first year when they had a habit of blowing out spark plugs.
If he has been showing the truck a lot, lots of idling, and very short drives when mainly cold, the mpg could be that low. Really wouldn't trust it unless you know how it's been driven since the last reset. If really interested, see if he will let you reset it and take it for a 30 mile spend and see what it reads.