There is a backflow valve built into the pump. You can order a new valve fairly cheaply.
But first try running the pump for a while. Do this without city water connected. This may clear any blockage.
There is also a filter next to the pump that may need cleaning.
Added: HWH Kick-Down Leveler; RoadMaster Anti-sway bars; Tornado Rotary Tank Rinser; Accede Backup Sensor Kit; cylinder locks (w/one combo lock); custom sink cover; Kenwood DDX 5706S front radio; Pioneer MVH-X5560BT rear radio; Oxygenics Shower head; ScanGuage II; Blue Ox Tow Bar w/RVi Brake 2; Winegard Rayzar Automatic TV antenna; Trimark Steel door lock; Dometic 310 toilet; whatever is next.