Harvest Hosts has wineries and farms willing to let you park overnight, in exchange for a visit to their gift shops: http://www.harvesthosts.com/

PG&E electric in California has campgrounds at some of their hydro-electric dams. First come, first serve, check to see if the park you like can accomadate an RV: http://www.pge.com/about/environment/pge/recreation/