Even if the fire doors are closed, they're easy to open and if anyone says anything just tell them that the door was open when you went past. We've never had a problem.
Parking off the premises is half the cost of parking near the ship, with a free shuttle included. Of course in involves another step in getting to your car but it's worth saving the money for us. We live in FL & are usually home by 11 AM.
It beats paying $20 per day.