I've gotten over 30 mpg. Good tailwind, keep the speeds modest, or elevation drop. I average >18 mpg overall, with lots of hill climbing.
I'm calling BS on this as well. My 5th gen 4runner, which is smaller, lighter and has a more efficient engine (though not by much) has barely broken 22 mpg on the highway, and that only happened on rare occasions. No way your Tundra is getting anywhere close to 30mpg except maybe for a brief moment when coasting down a 30 degree slope. I have doubts about you getting an 18mpg as an overall average as well.
But to see what people are really getting I just look them up on Fuelly. The 2.7l averages a little over 18 mpg. The Tundra is 14. I wouldn't really consider them equivalent engines though. The 3.5l and 5.0l are ~16 mpg.
The Ford ecoboost engines, both the 2.7l and 3.5l, are a bit more efficient than the Tundra's 5.7l v8. There is plenty of real world testing that validates that. Now, if you throw bigger tires, tow heavy or high payload, that fuel efficiency advantage starts to go away, as is the case for any turbo gasoline engine. Basically if the engine is under load, the turbo isn't all that more efficient over a NA engine. But for light cruising when empty, the Ford ecoboost engines get very good fuel economy.