FYI the “check valve” or backflow preventer is usually built inside the water pump. Sometimes this check valve will stick open (debris in the valve) and allow the shore (city) water back through the water pump which fills your fresh water tank to overflow. You can try to reset this water pump check valve by shutting off shore water and use your water pump again for a while. Then shut off the water pump and go back onto shore water again.......monitor the fill level on your fresh water tank to see if it is filling from shore water again. Sometimes this clears the problem which can be just some small debris caught in the check valve inside the water pump. If this does not work, then you can either replace the water pump, which is expensive, or you can install a stand alone check valve taking care to install it in the direction of water flow shown on the side of the check valve (water flows from FWtank to the pump; towards the taps in the coach). It helps to have a little RV plumbing experience though. If you do not feel confident doing RV plumbing you may want to have a RV tech do the work. I am attaching a Home Depot link to a Camco check valve (if my link works) but you can find them on Amazon also. They are a cheaper solution as an alternative.