How much power will my truck lose driving at 10,000 ft?

You will notice the power loss, and it is definitely noticable up high, but don't worry about the engine dying. The thin air means your V8 is going to run about 10 to 15 percent under its peak performance, which is expected at elevations around 10,000 feet. Since you'll be packed full of gear, that power reduction will feel magnified because the truck is already working hard to move all that weight. It won't feel like a huge, instant failure, but it will feel like the truck is just less willing to surge when you step on the gas, especially when you hit a particularly steep incline. As for the transmission, you probly won't need to worry about it hunting for gears excessively unless you start aggressive throttle input. Modern transmissions are pretty smart about load, but when the power output is lower, the engine will just require more patience from the driver. You should be shifting a little sooner than you would in the Midwest, keeping the engine in its sweet spot without over-revving, because you won't have the raw power buffer you are used to at sea level. Just treat those sustained climbs like a marathon, not a sprint.

For keeping the engine happy, the biggest thing is simply giving it room. Don't push the truck to its limit just because you want to make good time. Running the engine slightly under maximum load is the best way to keep things comfortable, especially when you're going through long days of driving and camping. Also, monitor your cooling system closely, even if it feels overkill. You just want to make sure everything is running at a steady pace throughout the trip. It's just about driving with respect for the altitude and the load you're carrying.

Took my gas rig up to Leadville a couple summers back, similar setup to yours. The climb is one thing, but what surprised me was the downhill side. Those grades are long and you'll cook your brakes if you ride them the whole way. I just locked out the top gears manually and let the engine hold me back. Barely touched the brake pedal after that. Also, plan your fuel stops more often than you think you need to. Stations get sparse up in the hills and I burned thru a tank faster than flatland driving ever did.

Agree with the 'less willing to surge' description, that's exactly how my gas V8 felt on a trip through the Rockies a few years back. What caught me off guard wasn't the power loss so much as the fuel mileage. I was stopping way more often than I do at home, sometimes filling up daily instead of every few days. On top of that, some of the little gas stations in the mountain towns are slow pumps, so build that time into your day. Carry a couple gallons of water in the bed too, cheap insurance and you'll use it camping anyway.

Update: got back from the trip and the truck did fine. The power loss up high was about what you all said, noticeable but nothing scary even loaded down. Biggest help was the tip about locking out the top gears on the downhills, that kept the brakes from getting sketchy on the long grades. And yeah, I was filling up way more often than usual.