The water filter is usually located very close to the water pump and city water connection. It's probably in a lower cabinet behind a removable panel.
I replace the filter once a year with one from a hardware store that's the same size and seal arrangement...2 for $12. Last time a checked the Water Pur replacement filter was $30 plus shipping.
Nights camped in 2013 - 55, 2014 - 105, 2015 - 63