you're absolutely right, there is a check valve on my freshwater pump as is the case with all of them, which stops water from entering the freshwater tank when you are connected to city water, BUT......I know there is no check valve on the "T" that leads out to the citywater hook-up from the freshwater pump. That's the part that baffles me. Why would water not run toward the citywater hook-up when running my pump.
There is only one reason or possibility that may be the answer, but it is only a guess. I'm thinking that as water is pumped from the fresh water tank out to faucets,,,,,, that as it passes the "T" where water would come in from the city water,,,,,there is a siphoning type thing going on where airflow from the city line stops water from running out that line. Hope any of this makes just a little sense???????