You only need one end open as the bottom is actually plugged anyway by the post that is in the bottom of the housing. The water comes in from the side and goes through the side of the filter material and up through the center and out the top of the filter back into the upper part of the housing to the trailer. These units work exactly the same as the unit I have in our house.
John & Deb
2011 F250 Lariat FX4 Crew Cab 6.2
2011 Flagstaff V-Lite 30WRLS
Days Camped 2013: 16 2014: 5 2015: 3