This map provides a pretty good guide, and I've found it to be fairly accurate as to orders of magnitude even if its data is dated:
http://darksitefinder.com/maps/world.html
To get truly dark skies you'll have to drive 5-6+ hours from SF. Much of the Sierras don't cut it anymore, and Mendocino NF gets lots of light pollution from the central valley, or too far north can have too much water vapor in the air.
Places I've been to within a 7-8 hour drive of SF with super-dark skies:
Central Nevada in the Humboldt-Toiyabe NF units (e.g., North Toiyabe Crest or Table Mountain), or pretty much any place in central Nevada.
Northern parts of Death Valley.
Warner Mountain range in Modoc County, CA.
Further afield:
Great Basin NP in Nevada.
Jarbidge Wilderness in Nevada.
South-central Utah (e.g., Capitol Reef NP).
Gila NF in SW New Mexico.
If you were to pick only one and don't mind a flight and a (super long) drive from Phoenix, Albuquerque or El Paso, Gila NF in New Mexico is the place to go. As I recall, driving along the northern fringe on route 59 I experienced probably the biggest lightless vista view I've seen in the lower 48 states, even compared to many wilderness areas in, say, Kings Canyon NP or elsewhere.
To get truly dark skies you'll have to drive 5-6+ hours from SF. Much of the Sierras don't cut it anymore, and Mendocino NF gets lots of light pollution from the central valley, or too far north can have too much water vapor in the air.
Places I've been to within a 7-8 hour drive of SF with super-dark skies:
Central Nevada in the Humboldt-Toiyabe NF units (e.g., North Toiyabe Crest or Table Mountain), or pretty much any place in central Nevada. Northern parts of Death Valley. Warner Mountain range in Modoc County, CA.
Further afield:
Great Basin NP in Nevada. Jarbidge Wilderness in Nevada. South-central Utah (e.g., Capitol Reef NP). Gila NF in SW New Mexico.