I agree with ktosv. For towing an RV, I favor the 6-spd transmission over all else assuming age/miles/price are similar. The first 4 gears of the 6-spd are worse than the 4-spd with 4.10. That means 2009+ for the 5.3L. If you can find a 6.2L equipped Yukon Denali, that allows for 2007+.
The longer wheelbase of the Ave is a positive, but the payload is lower for every Ave I have seen vs. Suburban and Tahoe. If you like the Avalanche, I'd recommend just getting a crew cab pickup. You get a higher receiver limit, more payload, and longer wheelbase.