Even of the Walmarts that do exist in the Bay Area, none of them allow it.
You see more homeless people in downtown San Francisco than anyone else, but they have now put up signs in many neighborhoods prohibiting RVs from parking on many streets overnight, even in places where parking spots aren't in demand. It seems that RVs endanger a housing market that makes it possible to charge two grand for a studio, and to literally rent out walk-in closets for $600 a month, all the while still allowing the renter to be ultra selective.