Most common cause for this problem is the check valve in the pump circuit allowing back leakage of city water into the FW tank. Shut off the city water, turn on the pump, open a faucet and let the pump run for a few minutes. In most cases, this will re-seat the check valve and you'll be fine.
I understand that there are some rigs that have a valve that allows you to fill your FW tank from your city water. If you have one of these, the valve may not be fully closed.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10