Solvang/Buellton other RV parks when Flying Flags is full?

There is a small state park in Lompoc.
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The only "state park" in Lompoc is La Purísima Mission; it doesn't offer camping, and it's closed on Thanksgiving.

As others said, your best bet is Cachuma,probably a dry site; this is the SB parks website. Click on "Camping Parks" and look at the Cachuma info. Jalama Beach is very likely full, and it's a haul out there and back (and out there and back) if you plan to run around wine tasting.

http://www.countyofsb.org/parks/