Hi,
With 4.10:1 rear axle ratio and only a 3 speed, the engine will be reving up pretty high at 55 MPH. At 65, expect about 7 - 8 MPG, and less when the camper is on it.
Also the GVWR is not very high. 10,000 GVWR - If I recall right. That is only about 2,200 pounds of payload, about what a F-250 from 2005 to present can carry with single rear wheels. And with a 4 speed, the F-250 will get much better mileage. And with the 'normal' 3.54:1 rear axle ratio, or 'optional' 3.73:1 rear axle ratio, even if driven in drive (third gear) the F-250 would get better mileage, add over drive and it will improve a lot more.
There have been a lot of changes and improvements to trucks over the years. The new 2015 F-150 with a 2.7L Ecoboost is predicted to consume only $2,600 in fuel, while a 2014 3.5L Ecoboost in the same truck (but 900 pounds more curb weight) will only get 18 MPG and consume $3,300 a year in fuel. So the new truck is predicted to use about $700 less a year in fuel, and that makes it worth more money than the 2014!
At 7-8 MPG, your truck will drive you to the poor house quickly if you are putting 15,000 miles a year on it!
Good luck!
Fred.
