The Surveillance Van is one of the seven vehicles which can be acquired in the first level of The Simpsons: Hit and Run. It is a black van with a camera attached to it on a pole on top of the van that can turn around and take photos. It is later revealed that it's just a pizza delivery van. It looks like a Ford Econoline or Ford Aerostar van.
Although it only appeared in The Simpsons: Hit and Run, a similar vehicle appeared in "Bart the Murderer" where, while Marge is voicing her concerns about Bart being involved in the Mafia to Homer, she asks why it would take over two weeks for a Pizza Delivery Truck parked out on their lawn to deliver a pizza to the door. It is implied that the "Pizza Delivery Truck" was actually a surveillance vehicle issued by the United States Government to spy on the Simpson family, as it cuts to inside the van and apparently heard Marge's question, and drove off as if they realized she was on to them only to be replaced by a "Flowers by Irene" (FBI) delivery truck.
The surveillance and spying can be done through CCTV Cameras but in that case, Bart and his Mafia contacts will get alert, which might cause a chaotic situation that would waste all hard work of security agencies, as they have not grabbed any vivid evidence yet.