Funny you should be considering what I just purchased....for the past 4 years I have been towing my very small 5th wheel with an F-150 with the small 4.6 V-8 - It towed it fine, my trailer is under 4K loaded....but I am considering a larger trailer in the near future and so a truck was the first thing - horse before the cart....
I found a very clean 1999 Extended Cab long bed V-10 with a manual transmission - it is a Super Duty with helper springs and by the Vin number rated to tow up to 12,000 pounds or so, you can say I now have overkill.
So far my around town mileage has been about 13.2 or so according to the scan gauge, have not checked it with the GPS yet - first full fillup was yesterday and the tank is large - 38 gallon and I put in 30 and it cost me $113.00. So now the truth begins as I am heading out this coming Monday with the 5er following me...
Had serious issues to resolve to get it hitched up - truck had been lifted by the previous owner 4 inches, and the trailer would not hitch properly and clear the bedrails, so had to order new U-Bolts and today I removed the 4 inch lift in the back, could not find shackles for the front lift, so that will have to wait...little front high, but she hitched fine with good clearance over the bed....only one minor issue, my small 5th wheel has such a short nose that the tailgate hits the landing gear when trying to hitch....have to back and clear the tailgate, get out, raise the tailgate and then run it in the hitch....not a huge deal, but she sits pretty good, trailer is a touch high but not detrimental.
The V-10 is a long running very powerful motor and I have been told the mileage hardly changes when under a load, so I expect to get about 10-12 with my small trailer which is 4 mpg better than my old small V-8 truck, so I should see a net gain...I would be happy to average 8-9 - that is just being in an RV!!!
I am very happy with the truck so far...more after traveling the next few weeks....but so far so good.
Jim