Everyone who sees my F-150 towing a 5th wheel says “it looks like a small truck to be towing a 5th wheel.” I have had this discussion many times; yes I have weighed my trailer and truck and I am at least 1,000 lbs. below my trucks towing capacity. The rear wheel ratio and echo-boost engine makes all the difference in the world, and I have been towing my 5th wheel for almost 5-years with this truck. I can go over mountain passes at 70mph without any strain whatsoever and most of the time it does not even feel like I am towing anything. I get between 9 to 11 mpg depending on how fast I drive. I like my rig combination very much and it really sets those with an F-250 or above back on their heels. Although I think I may benefit from some heavy duty brake pads.
P.S. My F-150 is an automatic and I usually use over-drive when towing. BTW, As a retired person I go out with my RV 2-weeks each month so I do a lot of towing, and I lived in my 5th wheel for 2-years, renting out my house, when I first got it.