Gary,

That slice of a moon on the door reminds me of OUTDOOR plumbing. The box is mounted on the rear deck don't ya know. I guess there wasn't any room in the underbays for a black water tank and must have been a concern for use of a macerator to fill an overhead black water tank! I just noted there is also a high placed ventfor removal of the taco gases or maybe that is to pump in fresh air containing oxygen.