Definitely sounds like you're sucking air, somewhere between the tank and the pump. As the water level goes down, the pump has to work harder to pull the water up. It's easier to suck air than water so when it got to that point, all you get is air. To find the tank, try and follow the gravity fill line to the tank.
2009 Fleetwood Bounder 35H
Ford V-10 w/ Banks