what they told me is that is has to be a 'house car'. Simply put, it has to have the amenities to support living in it (bathroom, kitchen, sleeping quarters, etc.). You will need to have it inspected by the DMV to have it considered and registered as a house car or RV. The requirements are vague and I suspect that was on purpose.
http://www.dmv.ca.gov/pubs/vctop/d01/vc362.htm
Mike