weather it is ducted or not, make sure your return inside the unit is sealed as it should be. take the filter off and look at the duct area just inside. sometimes the tape inside is not sealing where it needs to. if not, get some aluminum duct tape, the good stuff and seal it up. If it is ducted take a vent off at each end of the trailer and make sure you don't have any obstruction. For us, i had an issue with both, took care of them and my 15k unit takes care of the 33 foot fifth wheel. Also make sure you set your fan to on, not auto so it circulates the air all the time and it also helps it from freezing up. another option that helps is bubble foil for the windows.good luck