More than a dozen camper vans flanked the University Mound Reservoir in San Francisco’s Portola neighborhood Tuesday, spreading along University Street, past the antebellum senior center and alongside a row of abandoned greenhouses around the block.

To residents who flocked to a neighborhood meeting that night, the vehicles are a source of frustration and a vexing symbol of the city’s inability to solve its homeless crisis. To the people who live in those vans, they are a last line of defense against the streets.

“We realize these are people who are down on their luck, and we want the city to provide a safe place where they can have food, water, showers, bathrooms — stuff like that,” said Matt Lara, who lives near the RV encampment.

But Lara and others say their neighborhood isn’t a good site for a trailer park. They recently began circulating a petition to tighten parking restrictions, in hope of chasing the vans out. It has more than 700 signatures.

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Police and transit officials have a couple of legal tools to wield against the estimated 1,200 inhabited RVs that checker San Francisco’s streets — a 1971 ordinance against vehicle habitation and another that prohibits parking for more than 72 hours. Four years ago, the Municipal Transportation Agency banned these behemoths from parking overnight on 61 streets and roadways, and they’re effectively banned from other streets that have parking meters or time limits.

As the economic boom fills the city’s industrial streets with development and new parking restrictions, vans and trailers are moving toward the fringes. They collect in the Bayview, where homes are scattered and residents don’t have the political clout to get results from City Hall, or in the Portola, a neighborhood nestled between I-280, Highway 101 and McLaren Park, where blocks have long, uninterrupted curbs and parking rules are more lenient.

RVs symbolize a perplexing regional homeless crisis, but in San Francisco, the problem has largely fallen on the MTA’s doorstep, because the agency manages parking.

“The only way we can force a solution is to move these 1,200 vehicles around, until we eventually run out of streets,” said SFMTA board Chairwoman Cheryl Brinkman. But that doesn’t mean the larger issue goes away, she cautioned.

The Portola residents who packed a small room at Palega Recreation Center on Tuesday accused city officials of shunting the problem to their neighborhood. They say van dwellers gobble scarce parking, illegally plug into the electrical grid and dump gasoline or brown sewage water on the street.

Their comments seemed to rattle Supervisor Hillary Ronen, MTA senior analyst Andy Thornley and other officials who attended.

“When you have gray or brown water coming from vehicles, that’s a public health issue,” said Ronen, whose district includes the Portola. Fed up with the city’s lack of policy on RV dwellings, she said she will call for a hearing next week.

Ronen and others are incensed that the problem has migrated to the southeast areas, where households already struggle with poverty and the sentiment that no one at City Hall is listening. Several people who spoke at the meeting said the city would respond quicker if vans were clogging streets in wealthy enclaves like Sea Cliff and Pacific Heights.

“We’re tired of being a dumping ground,” Ronen said.

Although San Francisco has been grappling with vehicle encampments for years, the problem began simmering in recent months as more people adopted this form of transitory housing. Their reasons vary, said Jeff Kositsky, director of the city’s Department of Homelessness and Supportive Housing.

Some RV dwellers commute from the Central Valley to jobs in San Francisco, using their vans as de facto hotel rooms during the week, Kositsky said. Others are one step from homelessness — the vehicles they inhabit effectively serve as metal tents. And a small number of people use trailers and vans for criminal activity, such as bike chop shops, meth kitchens or sex trafficking.

“Well, nobody really wants to live like this, but what are we supposed to do?” asked a man climbing out of a Chevy van parked Tuesday afternoon in the Portola neighborhood. He declined to give his name. Another man in a mobile home on University Street yelled at reporters to get lost. A child’s bicycle was affixed to the roof of his van.

Kositsky’s encampment resolution team recently shifted its focus from tent cities to vehicle dwellers, who are dispersed throughout the city but are most prevalent in southeast neighborhoods and deep industrial pockets. As Portola residents fume about vehicles parked along the reservoir, a similar battle is flaring on De Wolf Street in the Outer Mission, where several vans are wedged on a block-long roadway between a stretch of BART tracks running alongside I-280 and a row of single-family homes on Alemany Boulevard.

Supervisor Ahsha Safai, who represents that area, is pressing for a nighttime ban on oversized vehicles — anything taller than 7 feet, or longer than 22 feet. But when the issue came before the SFMTA board last month, its members split. Three directors — Malcolm Heinicke, Art Torres and Lee Hsu — sided with Safai and the beset neighbors, while three others — Brinkman, Cristina Rubke and Gwyneth Borden — feared a ban would only push the vehicles somewhere else without solving the underlying social problem.

A woman who spoke at the meeting said she lives in a trailer with her handicapped son.

“Please don’t remove us,” said Gladys Odilia, 67.

In the Portola some residents are calling for permit parking on their streets, a solution that made Thornley, the MTA systems analyst, recoil. He said it would be contentious and costly to implement, even if it results in RVs getting towed.

“What we have is a parking and curb management puzzle, and a limited box of tools to solve it,” he told The Chronicle.

The rules are hard to enforce, Thornley said. Most vehicle occupants are savvy enough to relocate before the three-day parking limit is up, and if police want to impose the 1971 law, they have to prove that someone actually lives in the vehicle. Most occupants know not to answer the door when police knock, Thornley said.

As transit officials throw up their hands, Portola residents are growing impatient.

“I have young children, and I just don’t feel safe walking around with them when people are constantly coming in and out of these vans,” said Chrisoula Novo, who lives near the Portola Gardens senior home — a prime spot for RV parking. She noted that one of the van occupants allows a pit bull to roam the sidewalk without a leash.

Despite her concerns, Novo, like many of her neighbors, stressed compassion.

“We do feel bad,” she said.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan