It always amuses me when newbies are scared shitless by "serial killers!!!" and "robbers!!!". It reminds me of newbie backpackers, who always pee their pants over "bears!!!" and "snakes!!!"
Living in a van is no more dangerous or unsafe than living in an apartment or motel room. The vast majority of people in the US go their entire lives without once ever needing to "defend themselves" from any sort of attack. Statistically, THE single most dangerous part of living in vehicle is---driving it. You are far more likely to be killed by a simple traffic accident than you are from a serial killer or robber.
In 2.5 years of dwelling, the only incident I've had was when a couple of dumbass teens bought a BB gun in a Walmart and decided to test it out in the parking lot using my van. They got the shock of their life when I poked my head out the door.
If you are wondering what to carry in your van to "protect yourself", I recommend two pistols, a shotgun, an AR-15, a machete, a baseball bat, and a roll of barbed wire. And as a last resort, a W-80 warhead with a 200KT yield--it's the only way to be sure.