San Francisco has approved an ordinance that will prohibit recreational vehicles from remaining parked on city streets for more than two hours, replacing the patchwork of neighborhood restrictions with a uniform, city-wide rule. To blunt the impact on the roughly 1,000 people believed to be living in RVs, the measure establishes a permit system for short-term stays, a buy-back program for owners willing to surrender their vehicles, and guarantees of interim and permanent housing placements funded by the Department of Homelessness and Supportive Housing. Backers on the Board of Supervisors said the legislation is aimed at easing street congestion and health complaints while moving vehicle dwellers into safer accommodations. The Coalition on Homelessness, however, argues the two-hour limit amounts to an illegal displacement of residents who have few other options and is weighing potential legal action.