You'll find them on the desolate side streets of Potrero Hill and Bayview, along Golden Gate Park in the Sunset, and in other outlying neighborhoods: Campers and RVs parked in rows, their curtains drawn. Some are empty and sitting curbside because their owners don't have space to lodge them elsewhere. But others are full-time homes on wheels.

More than 250 people live in their vehicles, according to the city's latest homeless count. Nicholas Kimura of the Coalition on Homelessness says the real figure is likely much higher. And he says it's time for San Francisco to give RV residents legal parking. Other cities have designated sanctioned overnight lots for the "vehicularly housed," including San Diego, Santa Rosa, Ballard, Wash., and, soon perhaps, L.A.

"[San Francisco] has plenty of money and plenty of surplus property," Kimura says, adding that the city's current efforts to restrict street parking for oversized vehicles is ineffective at best, and discriminatory at worst.

Although it's illegal to inhabit a vehicle in San Francisco between 10 p.m. and 6 a.m., police rarely enforce that law. RVs were singled out for special treatment in 2012, when the Board of Supervisors limited the areas where vehicles longer than 22 feet or higher than seven feet could park. This made streets in the Sunset, Richmond, Bayview, and Potrero verboten for RVs, but this law has proved hard to enforce, too.

Since citations must be hand-delivered rather than tucked under a windshield wiper, camper dwellers simply refuse to answer their doors, according to the San Francisco Municipal Transportation Agency's Andy Thornley.

"Homeless advocates said it was an attack on people who lived in their cars, and the MTA said that's a valid point," he notes. "We're not in the business of social policy or housing, but we can't ignore the effects of these laws."

Like Kimura, Thornley supports the idea of city-sanctioned RV lots. The problem is determining which agency would oversee them. During the last two years, representatives from the SFMTA, the Coalition on Homelessness, and Mayor Ed Lee's office have met to discuss the issue, but have made little headway.

The SFMTA manages 19 parking lots, most of which are too small to accommodate more than a handful of vehicles. Church lots could be a better option, Thornley says, and the city's Interfaith Council has been invited to join parking talks. Another possible site is the property Caltrans owns beneath the I-280 overpass in Potrero Hill. Lava Mae, the mobile shower and sanitation company, could provide on-site services, although there are no specific proposals from anyone yet.

What is certain is that the SFMTA doesn't want to play a "gatekeeping" function. Vetting who gets to park in an overnight lot isn't the business of a transportation agency. Thornley calls parking "a very tender topic," noting that some neighborhoods will object to what's essentially a legal homeless encampment.

"It's a geometry of space problem," he says. "A problem of space versus sanctuary."