2013 Ford F250, 6.7 Diesel, Suggestions to improve mileage.

Like someone said, you might be stuck in regen mode. That's what I get in regen but after 18 miles or so it's getting 17-20 mpg. I've got 16k miles on my 2015 and I'm averaging 15.5 mpg since day 1, per the LOM. It's been a little better lately, almost 20 on the highway, unloaded. I got a scangaugeII to monitor RGN, EG2, EG4 and GPL to checkout my regens. Plug and play and it might tell you what's going on with your truck. Or back to the dealer I'd go.
Good luck
Rick