There is no firm estimate on the number of people who live in vehicles in Silicon Valley, but the problem is pervasive and apparent to anyone who sees RVs lining thoroughfares; not as visible are the cars tucked away at night in parking lots. Advocates for the homeless say it will only get worse unless more affordable housing is built. The median rent in the San Jose metro area is $3,500 a month, yet the median wage is $12 an hour in food service and $19 an hour in health care support, an amount that won’t even cover housing costs. The minimum annual salary needed to live comfortably in San Jose is $87,000, according to a study by personal finance website GoBankingRates. So dilapidated RVs line the eastern edge of Stanford University in Palo Alto, and officials in neighboring Mountain View have mapped out more than a dozen areas where campers tend to cluster, some of them about a mile from Google headquarters.