fresh water overflow

Hi We have a 2019 BDL. Have used it 6 times and connected to city water and shore power with no problem. I have a brass water pressure device set at 42 psi.  The last trip we returned after a few hours and noticed water flowing out of the fresh water fill port. I understand that there is a check valve located somewhere in the trailer connecting the two systems.  I  shut off the city water and drained the fresh water tank.  Turned on the city water and the fresh water tank began to fill again.  Any one else have this issue and where is the check valve located?  Thank you