I have stayed at the fairgrounds in Napa and it was adequate for a short stay. The only thing that made our stay worthwhile was that there was a cajun festival there the weekend we were.
Another really nice place is a little further out at skyline wilderness park. Reservations for electric and water, sewer first come first served with dump station on site.
If you have never been to Napa/Sonoma your in for a treat (assuming you like wine