Almost There said:Consider weight carefully before attaching anything to your rear door!
The doors and hinges weren't designed to handle any extra weight. Even those old spare tire carriers that were 3 point attachment (both hinges and the edge of the door) eventually pulled on the metal. That's if the metal didn't rust out first from the steel on steel and any movement from bumps etc.
I also have a 2002 Savana and I don't believe you'll be able to siphon gas from the tank.
BTW, my cargo carrier is a swing out one so that I can access the back doors easily. Instead of the expensive box that can be bought with the hitch platform I put a $65.00 Husky Tradesman box on mine.