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Your good mileage running through Death Valley was the result of the high air temps. The hotter the air, the thinner it is. I live at 1000 feet elevation. To get the same air density that they have at 10,000 feet elevation, the temperature around here would have to get up to 170 degrees F. If you know the air temperature and humidity during your drive through Death Valley, you could look them up in a density altitude chart to see what your air pressure altitude was.
You get better mileage at higher elevations because the air is thinner, which reduces your aerodynamic drag. Also the thinner air reduces the power output of normally aspirated engines and you have to operate at higher throttle settings with reduced pumping losses to get the same power output. Also modern engines automatically reduce the amount of fuel that they use since there is less oxygen in the thinner intake air to mix with the fuel. With older carburated cars you had to adjust the mixture as you went up in elevation to get them to run efficiently, otherwise they would run richer and richer as you climbed into thinner air. Then you had to remember to adjust the mixture back as you descended, otherwise the mixture would get too lean and the engine would become gutless when you floored the accelerator and also risked burning the valves.
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