Towing 26' with a 2003 Yukon SLT 5.3L?

You will be fine. I towed this EXACT combination for tens of thousands of miles, with zero issues. I had the same Tahoe, same drivetrain, including the rears, and another brand of trailer that was the same length, and within a few pounds on the dry weight. I bought the truck used, and immediately added air bags to the rear, and swapped the tires out for E rated LT truck tires. Having owned several Tahoes and Suburbans, I always get rid of the light duty tires and put the air bags on, it makes a huge difference. It is a great combination and should serve you well, including climbing hills, where you won't beat any diesels, but it will get you to the other side with no drama. Good luck.