When zombies are near a truck they will pile-up on the back. This occurs even if the player is near. They will switch targets, climb on the back of the truck and "forget" about the player.
I did this several times with this truck, in multiple locations and it worked. Also, had friends with the same experience. However, it isn't a 100% thing because I was attacked by a large group in Stary and they chose to eat me while ignoring the truck.