Winterizing/Drain Valves

I just got my 2006 3124KB a few days ago, and I have a question about the drain valves.

Three of my drain valves match (1 front, 2 rear), but one of the front valves is different (clear with a multi-turn valve) and nothing came out of it when I drained my tanks.

What is this "different" valve for? It comes into the TM just fore of the 40 gallon fresh water tank, but I can't tell what it's for.

TIA!