The door locks, trunk door lock, trunk lock, brakes, transmission shifting, engine shut off and god knows what else works on vacuum.
The engine has a vacuum pump as the diesel can't rely on crankcase vacuum (don't ask me why.. I don't really know). The vacuum from the pump is stored in a vacuum tank in the trunk under the rear deck (where the speakers, and first aid kit are).
That stored vacuum is used when the engine is shut off. That is to open all the locks.
Check for leaks to all the vacuum locking elements.
Now.. for the fuel door.. it's not gas door
:p just joking
The locking element is behind the right rear plastic panel in the trunk. Remove that plastic cover slowly (don't break things) and located the element. There should be plastic vacuum lines running through it connected by rubber vacuum hoses. The element is probably busted or the lines are leaking anyway once you get there you can figure it out.
Either replace the element or replace the vacuum lines.
Good luck!