See
This Photo Album for detailed Shower plumbing pictures with full text overlays detailing how these systems work.
This "
Mystery Port" is just:
A clever Shower Drain system
elevated
VENT / OVERFLOW PORT !!!
Here's the back side of the elevated vent OVERFLOW
So if worse comes to worst and the
Grey Tank (of which the Shower is it's only contributor)
fills up -
Any additional water attempting drain from the shower can
conveniently exit to the outsidpe of the RV via this clever port -
INSTEAD OF overflowing the shower basin and FLOWING ALL OVER THE INTERIOR FLOOR OF THE RV
Which, believe me, would damage all the interior panels, since after all, they consist of cheap particle "wood bits".
And when they get saturated with water from the floor up, they will puff out AND warp and get very distorted, loosing vertical rigidity and appear like a wax wall that melted a lot.
Here is the visual basic flow diagram:
So my friends, enjoy the free enlightenment!
My best
Fast Freddy