I just bought a 2008 National Tropical so I checked the manual for mine and I found this:
On mine I am sitting on top of the water tank and the plug is used to add antifreeze or bleach to sanitize. I am guessing that you would remove it to allow it to drain fully before flushing and refilling. I assume that we reinstall the plug before filling.
33' 2008 National Tropical on a Freightliner chassis.RVM#189