Mine did that on a long trip too. And the truck ran poorly when the light was on. I figured it was something heating up too much, since I had to run several hours on the interstate before it started happening. It scared me so I stopped and got a hamburger to let it cool off. Once back on the road (after a 45 min meal stop) the light stayed off and the truck ran fine. But then after about another hour of driving the light came on again, and it ran poorly again. I made it back to Missouri but never could figure out what that was.
I have attributed it to bad gas. But then again, I have not taken such a long trip since that time. That was a trip from Georgia to Missouri. Since then I have only gone on 2 hour trips here in Missouri, and the chk engine has stayed off. So, I am dumbfounded...
However; I am still not convinced it was a bad tank of gas...I guess next time I take a long trip I will find out.
My guess is that an O2 sensor was getting a lot of carbon blown by it, fouling it's reading. The truck acted like it didn't know what gear to go into because it was having trouble with the air/fuel mixture. It would "lug" and "bog down" on hills, staying in overdrive when it needed to downshift. I had to punch it to make it downshift like it was supposed to. To the uninformed observer this may sound like a transmission problem, but in reality it is a mixture problem; the PCM (computer) was not getting good info from the sensors, most likely O2.
Hope my info and experience with it helps you...
P.s. do a search here, as there are lots of messages about the chk engine light on long trips.