From the Fresno County site:

Can I live in a travel trailer?
California Title 25 does not allow the use of a recreational vehicle to be used as a
dwelling unit. The California Health and Safety Code defines a Recreational vehicle
as a motor home, travel trailer, truck camper, or camping trailer, with or without
motive power, designed for human habitation for recreational purposes.

Full document: http://www.co.fresno.ca.us/ViewDocument.aspx?id=60159