Watch your BIL's lips. If they moved or you heard those words coming from him, he was lying thru his teeth! 15-16 MPG towing anything over maybe a thousand pounds that's very aerodynamically styled with an F-350 (Ford doesn't make a 3500) V-10. He would have to be going downhill all the time and never on flat ground or any grade up or ever having a headwind at all and still towing at 50MPH max, period. DUH! What do you do once you get to the bottom of the hill or down to sea level? I gotta hear the answer!
Try 8 to 9 MPG pulling an average 28' to 30' 5th wheel if he is very lucky and drives under 60 MPH. The 6.8L V-10 is a very thirsty engine and feed it plenty of that liquid dino stuff and it preforms well.
Maybe he has a bridge he'll sell you real cheap too!