A car, van, bus, or otherwise four-wheeled vehicle that is decidedly sketchy. These tend to be found in parking lots at night, off in that one far corner in the dark. If you can't walk by it without getting the feeling some strung out tweeker is about to jump out of it and attack you, it's not a true sketchmobile.

A dirty, old Winnebago stationed next to a park at night, under a tree, with little flashes of light and coughing noises coming from the inside.
"Look! It's the sketchmobile!"