If the pump and lines are installed correctly, the only way that city water can get into the fresh water tank is through the pump. I suggest that you or your mechanic unhook the water line at the far end of the backflow preventer, hook it up to city water and see if any water leaks through. If it does, then you may have both a faulty replacement pump as well as a faulty backflow preventer, as unlikely as that seems.
Have you checked whether there may be a leak between the city water line and the fresh water line, especially if they're tied together at some point?
Gary Shapiro, 2011 Georgetown 29' Class A
Dash, Molly & Lily - now at the Bridge and forever in my heart
Shadow - 4-year-old Greyhound
Hannah - 1 1/2-year-old GSD rescue