I pulled a 2006 Chevy Colorado (same truck) about 70,000 and never had a problem. Since GM makes a big deal about being able to tow it 4 down in the owner's manual, I would expect no problem with the warranty, but a call to GM might be in order.
I wouldn't tow ANY vehicle without a brake system except in an emergency. Even with a 37,000 pound RV, that toad brake makes a noticeable difference in the stopping distance. In addition, in some (most) states and Canada, it's required by law.
ps: we just moved up to the Silverado last fall and the dealer loved the trade in with 27,000 miles on it. Yes, I did tell him it was a toad.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad