Are you sure that you are hooked up to the city fill? You may be hooked up to the Fresh water tank and water is coming out of the fresh water tank drain or over flow.
If you are hooked up to the true city fill (I have two hose fills, one for city and one for the freshwater on board tank), your water pump back flow prevent er may not be working.
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler