Fresh water tank filling internally

[RodeWorthy]

I have a situation where my fresh water tank is filling internally while connected to cg water supply. The tank will eventually overflow and spill onto the ground from the filler tube. The flow is low but steady and noticeable. What is provided to prevent the external water supply from entering the fresh water tank? I believe the Shurflo 5.7 water pump has check valves to prevent backflow. Is this the only source of the problem? I also have the 'noisy' hot water heater check valve problem. The Shurflo 5.7 pump was replaced in January because it would not shut off -- just go to a very low speed and growl away. Replacement corrected that problem. Has my pumped failed again?