Performance chip

I have a 99 with a 5.9L. I am considering a performance chip. Has anyone installed one and can give some advice. Is it worth the money? Will I notice the difference in towing? gas milage? I towed a 4500 trailer from Denver to Nebraska and barely got 8mpg. I know I won't get much better with a chip but considering the performace of the vehicle and the gas milage, I am hoping for something better. Eight mpg is ok if I can run 75 mph. I did 60 mph and got that poor of milage. Thoughts.

Hypertech makes a good programmer. I have a friend that bought one and is very pleased with it. He uses the "regular fuel" setting on his 5.9l. He reports a consistant 1-2mpg increase over stock programming. He has used the "premium fuel" setting a couple of times but did not see a great enough mpg increase to justify the expense of the fuel.
When you are running empty you are not pumping a lot of air through the engine so power lost to exhaust backpressure is minimal. When you are heavily loaded or towing, the exhaust backpressure can spike very high, leading to a substantial loss of potential power (up to 20hp) and mileage.
If you are looking for increased MPG while towing, the first place to start is usually a low restriction "CAT Back" exhaust. The next step can be Cold Air Intake or Reprogramming (such as the Hypertech).

Thanks neon_rt. I'll get to work on that.