Fred,
The four valves that you mentioned are the air tank drains, and like David said, your coach is equipped with an air dryer that removes the water from the compressed air just after the compressor and vents it every time that the compressor cycles. So if you start getting water out of the tank drains, you need to service your air drier. Most entry level diesel pushers do not come equipped with an air dryer, so as water collects in the tanks and air lines, sever maintenance issues can arise, especially in very cold weather.
The H2O purge button that you are referring to is part of the Racor primary fuel filter/water separator priming system. It is designed to dump water from the bowl on the bottom of your primary fuel filter if any collects there, assuming that the water sensor is functioning and has not been disconnected. If there is no water detected, it will not purge.
Gerald