Although people have heard of it, only some have actually seen it on their game, because most of them only know one way to get to the truck without Gameshark: getting a Pokemon with Cut by trading before getting the HM from the Captain.
Well, there is a much easier way to get to the truck, and it can be done any time if you have surf, ship leaving or not.
One way, without using a glitch, is to acquire Cut from the Captain, and then force all of your pokemon to die while still on the ship. You will wake up at the poke center, and ship will not have set sail.
Click this link to go to another article about reboarding the S.S. Anne. After you entered the harbor using the glitch or using the above method, surf to the right platform, and you will see the truck.
So why is the truck there? What's its purpose? Well, there are a lot of rumors about it, and none of them are true. The truck just does... nothing. We can only be sure that the programmers originally planned something for it, but just left it with no purpose as a remnant. Or someone could have put it there as a easter egg, a joke. Maybe that person just stuck it there as decoration but did not know that few players would be able to find it anyway.