We don't have 10,000 miles on our 250 diesel yet, but so far the mileage is dismal WITHOUT THE TRAILER. Often 14.
With the trailer we traveled from South Texas to Augusta GA, and then up through the mountains on the BRParkway, and through WVA to Ohio. Came home through KY, TN etc. We do not drive on interstates, enjoying more seeing the towns and countryside without trucks blowing us off the road. We averaged 13.4 for the whole trip which I thought was pretty good. Our 5.4 gas 2010 Ford averaged in the low 12s pulling the trailer.
Of course the dealer told me I'd get as much as 20 mpg without the trailer, and 15,16 towing. At 65 I learned not to believe car salesmen a long time ago.
It's worth everything extra I paid for it. I did lots of research and believe Ford is the very best HD truck on the market. We love the huge back seat for carrying extra stuff, and even though it's a 250, it rides every bit as well as the 150, which is great. Wife had a BMW until recently and I'd much rather drive the truck.
Bottom line: If you can afford the $60,000 plus sticker shock, you can afford fuel.
Matt & Virginia Kline
On Nueces Bay, near Portland TX
2016 F-350 6.7 Diesel (Overkill and loving it)