op wrote:
I am planning on taking an extended trip that will include time in Leadville, Colorado at 10,000 ft, and other western states. I am looking for advice from those who have done mountain and high altitude towing with a similar set up to what I have (see below). Will this be safe and dowable?
You'll be fine.
I pulled a 26' 5th wheel trailer all over the mountains in CO and northern NM with a '90 chevy 5.7 TB engine with a whoppin' 205hp/300 ft lbs torque. We made Wolf Creek pass/Monarch Pass/Red Mountain Pass/Independence Pass/Ike tunnel all in the 10000'-12000' range.
Your '08 FI 5.3 engine has around 300 hp and your towing a 6k trailer. I wouldn't add a thing to the truck other than fresh oil & filter/air filter/replace old rad hoses and new thermostat.
Just take your time and don't worry about staying up with the new gen diesels or the fast track lanes.