Another vote for the Yukon. Carpet and front seat recover is available and pretty cheap, the 8.1 is great (I have one in my 34' Super C motorhome, love it, and bought another to put in my 87), 4L80e is arguably the better transmission for the 8.1 in most stuff as the Allison soaks up quite a bit of power, and if you have a utility trailer, the burban will do anything a truck will do within reason while keeping more stuff (and kids, and kid's stuff) secure and dry. I've got an 04 Yukon XL Denali, the 6.0 is decent in the LQ4 form, but not amazing. If I had a 5000lb travel trailer, it would be borderline for long term. I'd much rather have the 8.1 for everything except gas mileage. You will get 11-12 highway, I'd guess, and dip to single digits towing, but I imagine you know that already, and I don't think the 1500HD will be a whole lot better without some work. A tune will help, I have also heard increasing the fuel pressure a couple psi on these is beneficial (you should have return style on an 03, and the regulator is probably adjustable). That may be due to E10 being the norm now but not accounted for then? Or maybe just better injector spray pattern? Dunno, and that is unconfirmed for me.
I'd cut your life expectancy in half for fuel filters and spark plugs, and watch for burned plug wires. If you haven't heard about the intake mod, you will soon, and let me know how it goes. I can't weld aluminum, and I'm too cheap to pay someone else right now, but it is a very intriguing option and I'll watch closely when you do that with a ZZ502 cam swap
