Pokemon Pokemon Message Board: No there isn't

The truck is in Red/Blue/Yellow, on the land to the right of the SS Anne. In Fire/Leaf there are only rocks in the water around the SS Anne.

And there was NEVER anything in/under/around/jigglypuff the truck. I repeat: NO MEW. The truck seems to have been placed there as a joke by the programmers, in an area they thought it was impossible to get to (since you need to Surf, which requires the Soulbadge, which you get long after the boat leaves). You can get there by leaving at least one unbeaten trainer on the boat when you visit the captain to get the Cut HM. Then go and lose to that trainer so you transport back to a PokeCenter. Now you can return to the boat after getting Surf, go right and see the truck and see that it DOESN'T DO ANYTHING.

You can do the same trick in Fire/Leaf, but there's no truck there. Just rocks.

The Mew crap was a rumor. I've see these various claims:

Use Strength to move the truck; Mew is under it. (The game doesn't let you use Strength on the truck. Nothing happens.)
Mew is in the truck. (There's no way to get in the truck.)
Get the keys somebody left at a slot machine in Game Corner and use them to drive the truck over a couple squares; Mew is under it. (You can't pick up the keys. There is no "TRUCK KEY" item. You can't get in the truck. You can't drive.)

While I'm on the topic of rumors: There is no Bill's Secret Garden, no Mew after you complete your Pokedex (even in the Japanese versions), nothing that happens if you beat the Elite Four umpteen thousand times without healing or turning off the power, no Pikablu, no Mist Stone in the Seafoam Islands or anywhere else, no Mew in Cinnabar Mansion (despite the diaries), no hidden "extra" Eevee-lutions, no Togepi or Marril in RBY, no Pokemon in RBY beyond the usual 151 (aside from glitches like Missingno and M-Block), no fossils other than Helix (Omanite) and Dome (Kabuto) and then Amber (Aerodactyl), no Bill's Secret Crack Stash, no Popemon, and Missingno/M-Block itself does not screw your save (just seeing it messes up the Hall of Fame [but only if you save after], and if you put it in a box when it's level 0 and then save, that box will be forever ruined).

There IS however one way to get a Mew in Red/Blue/Yellow (ONLY - NOT FIRE/LEAF) without using a cheat device. Don't fight the Youngster with 2 Slowpokes on Cerulean Cape or the Gambler just outside the Lavender entrance to the East-West underground path. Once you have Fly, walk in front of (at least 1 square away from) the Gambler and press Start right as the "!" appears over his head. Fly to Cerulean. Walk up to the cape and beat the Youngster (again making him walk to you). Now fly to Saffron and walk out the East gate (toward Lavender) and you'll encounter a level 7 wild Mew.