It may have lost it's prime.. Mine will do that if the fresh water tank does not have much water in it , say 5 or 10 gals, and the coach in not level. Water tank is flat and if coach is off level some , the water will go to one side , perhaps away from the pickup tube causing a loss of prime.. My solution is to keep more water in tank, approx 20 gals or so.
Some others may chime in here with other suggestions and ideas..