It is most likely sediment preventing the valve from closing completely. This seems to be a common problem with unfiltered water and RV's.
ALWAYS use a filter when hooked up to city water. Even when you fill the fresh water tank use a filter.
Chuck - Sparta, TN
2012 Jay Flight 22FB, 2 x Honda EU2000i
2013 GMC Yukon XL Denali AWD
EDUCATION is what you get when you read the fine print.....
EXPERIENCE is what you get when you don't.