The RV Park isn't a park so much as it is just part of a neighborhood.  We made a reservation by phone the morning we were going to pass through Sonora on a trip east on I-10.  The park manager met us on a street corner in the neighborhood a few blocks off the main street, and guided us in his truck to two FHU spots in an empty lot between two houses.  The hookups worked fine, and the rate was inexpensive.