There are many posts on FROG as to the Dometic 15K units failing to cool but still running. This is the first one I have read regarding a 13.5K unit. Are you sure that your A/C unit is a Dometic? One thing you can do is to go and buy a decent, about $40, infrared thermometer. You state that the air "does not push through the duct work in the ceiling". The grill of this unit, the white thingy on the ceiling, should have a way to "dump" air straight into the camper itself bypassing the ducted ceiling outlets. With the air dumping into the unit aim the red light of the thermometer into one of the little outlets for about 15 seconds and see what the temp of the air is coming into the camper. IF you get a reading of 33 degrees--50 degrees, this depends on the brand of unit and the BTU rating, which you stated was 13.5K. IF the air coming into the camper is close to the outside ambient air temp then you most likely have a refrigerant problem. Another IF, when you close the grill to divert the air into the ducting in the ceiling check ALL of the registers, where the air comes out, to see if they feel as if air is coming out. IF you can tell air is coming out check each register with your new thermometer toy. Draw a sketch of the registers from the front of the camper to the rear, number them and record the discharge air temp for each one. You could possibly have a blocked duct. When I bought my first nice camper some years ago the seller was honest enough to show me that the two most rear-ward registers had very little air coming from them. I took the registers off and ran a cheap plumbing snake, like a tightly wound spring, into the ducting and pushed out a lot of styrofoam. This was most likely where they cut the ducting into the styrofoam at the factory. These two registers worked good then.