Because bluegrass festivals are often at fairgrounds, we've found that they often have fine RV facilities with full hookups at very reasonable prices. In urban areas they can be a real bargain.
I just found this directory for the west coast.
http://www.rvfair.org
One might want to make sure they are not holding a hog slopping contest or banjo / accordion duets. Other than that they can be good places to stay.
Keith