1999 Chevy Tahoe Service engine light

I notice when I put load on my truck (towing our boat) that the service engine light comes on when the towing up a hill or when the truck is having to work a little bit. I usually loosen the gas cap and after awhile the light goes out. This time it's been 2 days and the light went out and then came back on. The other thing I do is put supreme gas in the truck and this also in the past has worked on making the light go out. Any ideas what I should do next, Thanks, Great site!