[beeje]

OP, The pipe you found is indeed the fresh water tank drain. The valve in the connivance center has 2 positions. Normal=when using city water or the water in your fresh water tank. If you are hooked up to city water there is no need to turn on the pump switch, the water pressure will get the water to your fixtures by itself. If you are not hooked up and are drawing water from your fresh water tank then the pump switch must be turned on to draw the water from the tank and send it to your fixtures.

By turning that switch to fill (the other position) (while hooked to city water) you are filling the fresh water tank (pump switch off)

With the switch in the normal position, and the cap off the fresh water tank drain, there should be NO water draining out. If you have water draining out continually you have a problem with a check valve at the pump.