I don't have a reference that specifies this but there are 6 gas stations in the Town of Leadville, Colorado. Leadville is the highest statutory city in the United States at an elevation of 10,152 ft. I'm not sure which particular one it is, but it's a good bet that one of the six qualifies.
"The City of Leadville is a statutory city that is the county seat, the most populous community, and the only incorporated municipality in Lake County, Colorado, United States. The city population was 2,602 at the 2010 census and an estimated 2,762 in 2018. It is situated at an elevation of 10,152 feet (3,094 m). Leadville is the highest incorporated city in the United States, and it is surrounded by two of the tallest 14,000 foot peaks in the state."