One line from the pump is connected to the FW holding tank, it is the supply to the pump. One line is the discharge from the pump. It could be 2 lines depending on the location of the pump and the trailer layout. The 3rd line could be the city water. Again depending on the layout.
You could have a leak in the supply line or a plugged strainer. Which means the pump can not prime itself. After checking for leaks and perhaps replacing the hoses on the input and discharge side of the strainer.
If you can remove the line from the discharge of the pump. Then run the pump and see if water comes out. If so reconnect the line, open all faucets and start the pump. Then close the faucets, one at a time. The pump should eventually stop as the pressure builds.
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