There are 2 choices, as I see it, listed in the Trailer Life Directory:
1. Contra Costa County Fair RV Park, has 24 full hookups, (925) 757-4400, $20/night last year. The fairgrounds are in Antioch, CA.
2. Sunny Acres Mobile Home & RV Park, 168 spaces, 73 w/full hookups, (925) 685-7048, $30/night last year. It is located just off Monument Blvd, about a mile east of Hwy 680.
Kirker Pass Road, on which the Pavilion is located, intersects with Hwy 4 about 4 miles west of the fairgrounds. The Pavilion is easily accessible from either campground, but is about 3-4 miles closer to Sunny Acres.
My choice, just because I don't care for the busyness of the Concord area, both freeways and city streets, would be the fairgrounds. I'm not familiar with either campground, but am intimately familiar with the Bay Area and its traffic. That's why we retired to the mountains!