The Fairgrounds are easy to find, and when we camped here on a Saturday in April 2016, we had plenty spots to choose from. The site we choose was not very level, so bring your leveling blocks. We stayed on a grassy field, I am not sure what it will look like in the summer.
Hookups are electric only, but water is available on the Fairgrounds.
There is a check out time, but not a check in time, which is great when you arrive early! You can just self register. The California State Mining and Mineral Museum is at the Fairgrounds, well worth a visit.
As it is reasonably close to Yosemite, it is a good and affordable spot to stay for a little longer. Unfortunately we had to go back home to our jobs after the weekend. We did visit Hornitos on our way back, a really cool little ghost town nearby.