Check engine light comes on towing a trailer with 2007 Chevy 2500 HD

there are a number of reasons for this light to illuminate~ is going to be with the emissions mostly~ the evap system must breathe easily...with that added load is constricting proper combustion~....just a theory,

If the light came on then it stored a code, what is that code?

try putting fuel injector cleaner in it ,could be as simple as that , note if you get stabletrac , reduced power service light it will use usually make that code go away within 10 minutes