According to the BTTF wiki, Hill Valley was filmed about 6 miles west of Chinese Camp, near Sonora, CA. And certain buildings were in various other locations, such as Pasadena, CA as noted near the bottom of the Hill Valley page.
The Hill Valley page says in-universe, the town is near U.S 8, which is in Wisconsin, and Highway 395, which is in California. There's an intersection of U.S. 6 and Highway 395 near Bishop, CA. There's also an Interstate 8 way south between San Diego, CA and Casa Grande, AZ (about halfway between Phoenix and Tucson).
Grass Valley, CA is a real town, quite a bit north of the others. There an Interstate 80 near Grass Valley, which is similar to U.S. 8, but may be me grasping for connections that aren't there. In-universe, it's supposed to be about 12 miles west of Hill Valley.
Lone Pine, CA is also on Highway 395, and there's a Lone Pine shopping mall in-universe.
Here is a map with Lone Pine, Bishop, Chinese Camp, and Grass Valley all marked as waypoints.
Also, this website has information about visiting a lot of the sites used to film the 1885 version of Hill Valley, which could be pretty cool. Apparently most of it is accessible to the public.
The map posted by rand al'thor has a number of locations, but I can't find any references to most of them out of universe:
- Hill Valley
- Haysville (on the road southeast of Hill Valley)
- Shonash Ravine (bridge west of Hill Valley)
- Carson Spur (the road leading to Shonash Ravine)
- Brides Pond (northwest of Shonash Ravine)
- Grass Valley (west side of the map)
- Indian Galley/Gully (can't quite tell, just east of Shonash Ravine)
I see a few other words, like "trail", "city", or "valley", but I can't make out the important half of the name.
As pointed out by other comments on this page, and the Snopes article, it's definitely set in California somewhere, but I don't think it gets any more specific than that.
Maybe it used to be one location in the past, but after all the time-travel, some residual spacetime artifacts caused the town to get scattered across California?