It's adjusting a bit and may not have totally learned the new program.
Your truck may not like the eco tune. Are you running the 87 octane tune? My '07 hates it. I get better mileage with the 91 octane tune, especially on road trips, where I will be around 14.5 to 14.8 mpg. Around town, not so much. But hey, I run mine hard. I'm lucky if I see 11 mpg around town. But heck, I like hearing the Magnaflows roar and the slippage of big 35's from time to time. I surely didn't buy this big lifted MegaCab with a HEMI for mileage!
With the 87 octane fuel and program, my truck was pinging and I felt it was down on power which means, knowing how these PCM programs work, the knock retard program detects the slightest bit of knock and pulls timing and fuel. It's very inefficient when that happens. My mileage, even on highway runs never got above 12.5 or so.
I'd run out the tank you have, try a higher octane (89 or 91?) and install the appropriate level tune based on the octane rating of the gas you buy.
Don't install a 91 tune and run anything less than 91 octane.
Once you have switched tunes (out with the 87, in with stock, out with stock, in with 91 tune or whatever you choose) and have the right octane, go run it and drive it hard for most of a 1/2 tank or more.
Then you can back off and check mileage based on how you normally drive.