I think he means on the side of the trailer AT THE FRONT. He says by the LP grill port, so near where you pull out the LP line for the grill.
Still need to know which model he has.
I found this pic of a 21DK; It says it's the black tank flush, near as I can tell.
Jason, unless someone smarter than me chimes in, this is for hooking up a garden hose and flushing out your black tank after you've drained it.
Actually I think this is on the street side by the shore power hookup, so may not be what he's talking about.
Jason, we need your model number and a pic if you can post one.