Hi, I have a Coleman Evcon C3QA-036U-B in a Double Wide. When we put the thermostat on auto, the unit turns on and off fairly quickly ( id say every 15-20 min) but the air never really gets warm, it just stays cool. Any one know what the problem could be?

Check to be sure that all duct work under the trailer/mobile home is connected. It can come apart and cause this problem especially the return air jumper.

It can be the duct work yes or it can be that thingy that looks like a hockey puck about 30 dollars to replace. Its called a pressure swiitch.. They have been known to fail on colemans. You can unscrew the one you have pull off the hose take a long pin to poke into the holes (clearing any blocage.. pop a pin in on the switch and on the furnace itself.. put it all back together. And see what happens generally can get clogged up by calcium deposits or dust