If you mean the valve in the docking center it is for pulling antifreeze from a jug. Look for an unused hose (1/2" probably white) near that valve.
When you find it put your finger over the end and flip the valve. You'll have to have a faucet open first. If you have the right hose you will have a suction and have found the winterizing hose.
Stick that in an antifreeze jug (pink RV antifreeze) and that valve will allow you to pump antifreeze to each faucet. Find the closest faucet and open it, run till pink. Do the same for the next faucet and so on. Don't forget to run the toilet and shower, both inside and out.
