According to the myth, if you went to the truck near the S.S. Anne and used Strength on it, the truck would move and a Poké Ball containing a Mew would appear where it was.
Why doesn't it work?Edit
The truck is just an easter egg. And no, the keys at the Game Corner will not open it. No matter how many times you use Strength, the truck won't budge. Also, you cannot slash the tires using Cut, even though that wouldn't do anything to move it.
The information was from a YouTube video involving the truck (the link to it is in the external links section).
A video involving the myth.