LOS ANGELES — Every day, Diane Butler and her husband park their two hand-painted R.V.’s in a lot at the edge of Venice Beach here, alongside dozens of other rickety, rusted campers from the 1970s and ’80s. During the day, she sells her artwork on the boardwalk. When the parking lot closes at sunset, she and the other R.V.-dwellers drive a quarter-mile inland to find somewhere on the street to park for the night.