Big Red,

does your friend add Cetane boost?
transfer case or front hubs stuck in 4x4?
different gear ratio compared to your friends?
different tires and/or tire pressure?

Could your t-stat be stuck, causing your truck to not warm up correctly
and thus burn more fuel? The dash gauges are not of much value.
An ODBII gauge will give you exact temps and immediately find a bad t-stat.
Engine coolant of 190* is a sign of a working t-stat.

Edit: By the way, I regularly get 17-18 and I got 21 MPG round-trip FL - NC (@70-75MPH).
I routinely go 700 miles on a tank (44 gallon tank in an Ex)
I filled up last week: 703 miles on 41.6 gallons (16.9mpg) no long hi-way trips on this tank.