Alice, welcome to the forum. If you're connected via a hose to a water supply, that's all you need to do. If, however, you are not so connected, and you wish to use the water from the tank, you need to switch on the water pump. There are usually a couple of switches. One will be by the kitchen sink, perhaps on the wall to the left of the sink, and one may be on the wall above and to the right of the washbasin in the bathroom. They act like light switches at either end of a long passage in a house, in that either switch may be used at any time. On many trailers, a small red light will illuminate on the control panel above the front couch when the pump is activated. The pump will only run (noisily), when a faucet is opened.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."