While pulling a trailer, which was within.

While pulling a trailer, which was within vehicle towing capacity, the Tahoe felt sluggish while pulling up an incline. Eventually, "Service Engine Soon" came up and the vehicle lost power. I was able to maintain 60-70 mph prior to the vehicle losing power, but only able to get to 35 mph afterward. Once trailer was unhooked, the vehicle ran fine. Took it to the local shop and they advised several codes came up. They checked everything out and all checked good. The codes were cleared and the Tahoe ran fine, until I hooked up a small utility trailer. I took the trailer to the shop to have it checked out to make sure it was road worthy(didn't want to put anything on it if the axles were about to fall off). The Tahoe ran fine on the way to the shop. But, after I left the shop, it started to act up once I got on the interstate. No engine light or anything this time, just a loss of power, or hesitation. What could cause the vehicle to have issues while pulling a load, but run fine otherwise?