2014+ GM1500's got a substantial power increase, along with mpg improvent. ALL got the 6 so auto. A few of the 12/13 5.3's got the 6sp, the rest have the 4sp.
You don't have ALL that much more power than I do with my 2014 4.3. I've pulled a few trailers in the 6-8k range. They were not box dye RV trailer. Generally speaking, I'm not lacking in power.
At elevation losing 3% every 1000'. You may notice in the 5-6k' range. Gear down and go slower.
Overall you will be safe. I would bet even same at the end of a trip. A trailer that is more aerodynamic vs not, would be a better choice. Make sure it has smooth sidewalks, not corrogated aluminum. That is worst option for wind drag, extra HP needed etc. Yes it's lighter....but!
Marty