An RV water pump that has not been used in a while, whether because the RV was being stored or because it's a newly installed pump, must be primed before use. The process is easy, however, accessing the pump is notoriously difficult. Pumps are often installed under the bench seats or in cabinets as near to the water tank as possible.
Flat head screwdriver
If your pump is not easily accessed, trace the water line to where it first connects to a sink (almost all RV water lines will run to your kitchen sink first) and disconnect that line and use it as if it was the rubber hose attached in the steps. However, pour about a gallon of water into the line instead of a jar full so you do not create an air trap in your line.
If you prime a pump after winter storage and you have used antifreeze in your water lines, leave the faucet open and the pump running until the water is absolutely clear. Antifreeze left in your water supply can make you very ill, make sure it is completely clear.
Locate your RV water pump. If you do not have a manual to consult, look under either the bench seats or in the cabinets nearest the water storage tank. If this fails, open the cabinet underneath your kitchen sink and trace the water line back to the pump.
Disconnect the water outlet line from the RV water pump by turning the screw on the attaching collar counter clockwise until it is loose enough for you to pull the hose off. The outlet line is the clear plastic line that comes out of the water pump and goes into the RV to feed sinks, showers and toilets.
Slip the collar off the outlet line and on to the end of your rubber hose. Attach the rubber hose to the outlet spout on the RV water pump by screwing the clamp down.
The rubber hose should be the same size as your water line and long enough to extend without stretching or kinking from the pump to you.
Fill a jar with 12 ounces of water (you don't have to be exact, about a soda bottle full will do).
Hold the hose you attached to the outlet spout up in the air so it is above the level of the pump. Pour the water into the hose. Place your thumb over the top of the hose and turn the pump on. Give it a minute to build up pressure and then you should feel the water against your thumb. Turn the pump off.
Disconnect the hose from the pump and reconnect the water line. Open one of your faucets and turn the pump on. You should see water running in a few seconds. Turn off the faucet and the pump.