This from my 2014 Chevy Duramax Owner's Manual:
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Do not tow a trailer during the first 800 km (500 mi) to prevent damage to the engine, axle,or other parts..Then during the first 800 km(500 mi) of trailer towing, do not drive over 80 km/h (50 mph) and do not make starts at full throttle.
I found this information after I towed the trailer, and I was over 1000 miles then. oops! But, I also never towed more than 50-55 mph though, as the first 2 or 3 camping trips we never drove interstates.
Truck now 3 years later, and seems to be just fine.