The choice of the catering vehicle is made depending airplane doors height.
Since the default 747 is already properly configured in the GSX internal database and doesn't require customization, the only possible explanation for having a small Van instead of the proper catering vehicle, is that you tried to customize the airplane and made some kind of error, like moving the door too low.
You can use the Reset button in the GSX airplane configuration editor, to restore the plane configuration to the one that comes with GSX.