Thx for the comments. Got home last night after a 5 hour drive with the truck and no TT. Drives great except for the tires as below.
The truck was a rush purchase as we just had to have a replacement or it was no more camping in time for the next trip and no way to get home except for a rental car. I had zero time to research anything or even inspect the truck. My BIL runs an auto parts shop and is well-connected to the local car/truck community. The guy, and friend of BIL, that we bought the truck from buys used trucks, fixes them up and resells them for a living so we know it's a pretty decent truck.
The 2500HD came with over-size 265/17 M+S LRE off-road type tires, whereas OEM width is 245. I didn't like the way the tires felt at highway speeds (up to about 80 mph). Haven't checked the tire age yet but a new set of Michelin LTX or similar is on our immediate to-do list regardless.
I see now that LRE in 17" is 3195 lbs so no problems at all. If 17" makes the truck lower to the ground than the old 18" and easier to get in, that's a bonus, lol. :)
Last two trucks have been Ford and never thought I'd get a GMC but I like it so far. A lot quieter than the old V10 F250 and seems to shift smoother.