The truck is not a decoration! It is a useful thing! I know how to open it! It is not that hard! People say that there is a portal to go through! It is actually to get a Mew. The truck belongs to the captain of the S.S.Anne.
1. You must catch all of the Pokemon including a Mewtwo. (No Mew)
2. Go get the degree.
3. Go to the Game Corner.
4. In one slot there will be some keys. You can get them once you have all Pokemon (No Mew).
5. Go to the truck.
6. Press "A" near the door and it will open! A mew in a Pokeball will be in there!