mountains with 6.0 gas

My 2015 Chevy 3500 at work has a tool bed on it and weighs about 9,000 pounds with it. I had a mini-excavator on my flatbed trailer (about 11,000 pounds) behind me going up Afton Mountain one day. With the gas pedal flat on the floor it would only do about 35 MPH. It got me up the mountain with no problem, just a bit slow.

Good advice from fanrgs above...
You won't win any races and you will be able to watch you gas gauge head toward empty....

Have a good trip.

thanks I have a 2013 with 19000 miles trans and brakes good

You will go through the Eisenhower Tunnel at 11,000 feet and over Vail Pass at 10,600 feet. Your truck will be starved for oxygen and lose 1/3 of its horsepower at those elevations. So you will likely not be able to stay at the 65 MPH speed limit going uphill. But neither will a lot of other RVers and truckers, so just do the best you can--it is only 15-20 minutes of a 4-hour drive.

More importantly, how are your brakes and transmission? They will get a harder workout than the engine. But be sure to use your gears and not your brakes on the downhill side or you might be forced to use one of the runaway truck ramps. That would not be any fun with a trailer!

No issue I see.