Regardless of ALL factors [Economy Mode, MDS, flat terrain, etc...] you should be getting a LOT better than your posted MPG on the highway/interstate.
That's just pathetic!
Something's clearly wrong somewhere. Pay your dealership a visit ASAP.
Truck was sold today [5/19]. I got an offer I couldn't turn down. I had no more need for a pickup so I cut it loose. About to purchase a '14 Jeep SRT8 this week.