My TT came with one. It was under the kitchen cabinets with the fresh-water pump. I removed it and use it outside the trailer. As someone posted above, mine froze and broke, but landed like a tumbler, and didn't spill its contents, (lucky me...) That's why I moved it, so I could make sure everything drained and pump antifreeze throughout the system. I bought a new one at Lowes for about $30-. I already had a dozen or so of the filters because our last home had a well, and these filter cartridges fit that filter system... When you change the cartridge after camping off and on for a year, you'll see why it's a good idea to use a filter.
J & D in Lovely Weatherford, TX
2016 Jayco White Hawk 28DSBH TT
2014 Ram 2500 Crew Cab 4X4 6.7L Cummins