Leaking Gas Tank

[anubisds]

Hello all,
After the hottest day of the season my trunk began to smell like gas. Its definitely in the trunk, not the engine bay. Theres a brand new mercedes regulation gas cap on it and I havent moved or messed with any of the hoses. The dealership says I need a new gas tank. Is it more probable that the O ring on the fuel-screen has deteriorated or that one of those hoses running from the gas tank to the fuel pump assembly has cracked? Any other common/possible cause?

Thanks for the suggestions guys!