This depends on two things: one is position of the slot -- when the slot is in a horizontal position, trunk and gas cap will stay locked when you unlock the doors. In the vertical position, the trunk will unlock when you unlock the doors. Also, locking the trunk to either position will lock the doors.
Second, can you unlock the trunk at all? The "valet" key will not unlock the trunk. You'll have to order a master key from MB if you have only the valet key. It is fairly expensive.