The Tahoe is not the best in terms of gas, but honestly, it depends more on how you drive than the engines truck itself. You can easily get 15mpg or better on the freeway, and 12mpg or above in the city as long as you're not racing to every light or to every offramp. That said, you probably won't get better mileage than the older Chevy pickups, as the trucks got newer 2000 and up, they were made for more power rather than more mileage.
Chevy trucks are very reliable, especially under crappy conditions. It seems like everyone is talking Toyota these days, well lets see what happens when you take a Toyota Tundra and stick it in a real work environment over the course of a few years and see what happens to it.
You never see Tundras and hardly see Dodge Rams on construction sites. Why? Because they won't last under the stress of the American Construction work environment. This is why you almost only see Chevy, GMC, and Ford trucks on construction sites. These are trucks that are meant to be kicked around and abused.
I currently drive a 94 and a 95 Suburban both with more than 260,000 miles on them. The 95 runs great and has 95% of it's original compression. The 94 has 2 dead cylinders because my sister forgot to change the oil. But, I just got back from a cross country trip in it, and despite having 2 dead cylinders I averaged 15mpg and I had no problem climbing the mountains.
So overall GM pickups are about as reliable as you can get, and the 2004 truck line is no exception. If you can afford the gas pick one up, you won't regret it.