Fred,
Not sure what brand water pump you have. If it is an Aqua Jet, the check valve is in the head and it is not replaceable. However, they will supply an additional check valve that snaps into the pump's output port. A few years ago I had a similar issue with a bad check valve. I found that if I was hooked to park water, the fresh tank would slowly fill as water leaked past the faulty check valve. When switching to the fresh water tank, the pump would burp every few minutes.
Steve