Thanks again for all the great responses and good information. :)
My wife and I have been talking it over and given the payload and hitch limitations, I am going to look a little more at the older units. Her concern is safety and being overweight, as we got out of a Sierra 1500 initially for those reasons. If I can find a gently used 8.1 that I can use primarily as a TV, that might be the best option. I'll still have three vehicles, but will be money ahead since it will cost less than my current Duramax to purchase.
Just to see how things handle, I'll be taking my Dad's "new to him" 06 2500HD with the 6.0/4.10 combo up to Sunset Point this weekend towing my trailer. It's a good climb out of Phoenix and should give me a good idea if that combo would be bearable in the Sub. Obviously the 8.1 would be better, but it can't hurt to test it out.
I'll be sure to update the post once we have made our decision.