Sounds like you're sucking air instead of water from the fresh water tank.
Take the line loose from the discharge of the pump to make it easier for the pump to prime.
You might need to connect a hose onto the suction side of the pump to get it to pick up water after 4 years of setting idle. Once it's primed, connect the suction side back to the water tank to see if will pick up any water from it.
Can you put a clear hose on the line from the fresh water tank and use your mouth to draw water up the hose to verify the line isn't cracked and drawing air instead of water?