This week I drove around with the truck full of stuff just to see how a little weight would change things. I have basically everything one needs to camp and a whole lot more, along with a full cooler in the back. I did notice a bit of a decrease as I usually get 310-330 miles on a tank. This week, my light came on at 283 miles, and now I'm at 300 with the needle still a bit above the E-line.
All this on a bone stock 2001 4x4 Sport with ~70,000 miles. I think it all comes down to having a light foot and not setting a cruise speed too high. In a 55 zone, I won't exceed 62 and in a 65 zone, I won't exceed 72. I've seen dramatic differences from just this one change. Good luck guys.