2015 350 XLT with 3.7 L/3.73 rear.. horrible mileage

First of all, we do live in WV so there are some hills around here, so I expect *some* decrease from the 19 mpg highway estimate. However, I'm getting a hand-calculated 13.7-14.2 around here most of the time with an 80/20 split of highway/city, respectively.

Van has 35,500 miles on it and the only thing that's not stock is the tires... currently running 225/75/16's instead of the 235/65/16's, so it's a 4% increase in tire size... that only equates to around 85 rpm at 70 mph, so it's not much. I had to change them out after we'd had the van less than 2 weeks because the stock tires started showing cords and throwing chunks of rubber off of them at 34,500 miles.

Anyway, I talked to a guy from church today who has basically the exact same van as us with the same amount of people to haul (and drives the same route to get home), and he's getting 17-17.5 average, so around 20% better mileage. I have no idea what's going on with this thing.. There's no CEL showing, so it doesn't appear to be something so big the computer is fussing about it, but I can't imagine that mileage being anywhere near normal. If I keep it at 45 mph around here on the side-roads I can get a decent mileage reading on it if I reset it and run a few miles, but once we hit the highways and actually have to GO anywhere, the mileage drops like a rock. I don't ever even push it past 72 or so, either, so it's not like I'm running it hard.

Any ideas? Going to try to get it into the dealer to run a diagnostics on it for the awful mileage this week, so hopefully that will shed some light on it. I'm just about ready to trade the frustrating thing in for an ecoboosted van, though, as at least those have a decent amount of torque to get up the hills.