Do you live in Seattle? I'm asking because in Seattle, at least, you can't park on any given block for more than three days at a time. You can use this link to report the camper anytime it's parked in a location for more than 72 hours. By law, they'll have to move the camper within three days of getting a notice to move. Unfortunately, they only have to move to another block, but, if you keep up with it enough, they might get sick of it and move on. You have to give parking enforcement the make, model, and license plate number of the vehicle, so I recommend writing that down for safekeeping.
If you don't live in Seattle, try looking up what the law is for how long a vehicle is allowed to park on the street.
How dangerous do you think those neighbors truly are? If you think their actions will stay limited to vandalism, and if reporting the camper doesn't work, maybe you could band together with your neighbors to drive the motherfuckers away. Take turns reporting every possible incident to the authorities, keep an eye on each others' property to catch vandalism in the act, and maybe even consider setting up surveillance cameras.