tbiker99,
Now that you know what it's for, make sure it is open when you fill your fresh water tank. When the tank is full (probably slightly over-filled) you may see water come out that drain.
I put similar valves on ours because I've had incidences where I'd fill the tank, hit the road, and 2 hours later, there was still water siphoning out of the vent hose. One of these days, I'll fix that! LOL
In the meantime, I put a valve in line. Now if I fill the tank so the water drains out that valve before hitting the road, I'll close the valve down until it's just a very slight trickle. This limits the leakage yet can provide for air to enter the line should I use the water pump. If that valve is closed you will start creating a vacuum in the water tank, which might cause damage, and you will probably notice a decrease in water flow at the faucets. Usually, you will notice this when you go to use the bathroom in the middle of the night! LOL
Glad to hear you were able to figure it all out.
~Rick