Need Help Please!

I pull my 9000lbs trailer everywhere with me all over the country. I was traveling down I-81 in VA and going up and down the mountains with no problems. On the final stretch, there is a 7 mile downhill descent with a decent downgrade. I was coasting around 60mph for about 1 mile and then check engine light came on and the message said that my truck was in reduced power mode. I decided to just coast it down the other 6 miles and went right into the NC rest stop. I let the truck sit for around 15 minutes and started it again. The reduced power mode turned off but the check engine light was on. It was a Sunday and still had 1.5 hours till my destination so I decided to just keep going. The truck seemed to operate normally for the rest of the trip and I got to my destination, dropped the trailer off, and was going to head to Autozone to get a code reader for the check engine light. However, after I restarted the truck at my destination, the check engine light was off and still hasn't come back on. I am not sure what to do? I tow a 9000lbs camper everywhere and can't have the truck going into low power mode on the highways! I also can't afford to just start replacing parts that might not be the problem either. Any advice would be much appreciated.