Same opinion here. I'm at 12 city, 13 mixed, 15-16 highway with 33's, 2.5" lift, ALL the armor, Tacobox crawlbox and a roof rack full of gear.
Probably at least 500-750lbs over stock.
I do take advantage of cruise control when i'm going faster than 40-50 on the highway, and I keep my speeds under 65 pretty religiously. Also, I keep my tires well-inflated.
I usually have a range of about 350miles. Then again - I have to put this out there - I don't care? I didn't buy this truck for the fuel mileage, we bought it for the capability/durability/reliability, I'd much rather have an engine known to go 100-200k miles with zero issues than one that gets 20-25 mpgs but also has known faults (our 2.7 Ecoboost, in our F150, for example - we're babying it, driving it lightly and are pretty much ready to chuck it at/before 100k)
some thoughts:
Maybe a brake caliper(or calipers) is/are dragging?
Maybe a bearing is bad, causing resistance?
Tire inflation?
Reset trip meter at each tank to get a new reading?