Dark Sky will always be where there is less atmosphere to look through, so finding something with elevation is going to be a good beginning.
Away from jetstreams that carry pollution is another consideration, as there will be less reflective metal particles in the air. Also, while you will get spectacular sunsets and sunrises, during dusty periods, such as harvest seasons, the worst time is going to be during high periods of forest fires. Interesting enough, due to the refractions, you will see higher solar harvesting during these Smokey fires.
Ft Davis area of Texas is known as one of the darkest areas in the country, for many reasons. Personally, my recommendation would be the Wind Rivers of North Western Wyoming, just go to Pinedale or Bondurant and head up. Be sure to go during the time of of a new moon and less rain events. Such as July and August, which is typically the busy time of summer. While these would be boondocking locations and not campgrounds. Hopefully that is what you are looking for, since you posted in a Boondocking, drycamping forum.
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