The other suggested reasons for the water pump to keep running are good ones. There are a couple more. Air in the system is one. Another is if the pump, due to some mechanical problem, is unable to bring the water pressure up to its automatic shutoff level. The latter can be checked by turning the pump off and then hooking up to external, well-pressured water so that the RV's water system is at a pressure higher than the pump's shutoff pressure. Then turn off the external water and turn the pump on. If it does not start running again, open a tap until the pump does run, turn off the tap, and see if the pump continues to run on. If so, the pump could well be faulty. Obviously, we have had this experience!