Thomas,
The evap coil is under that styrofoam cover and mine is an insulated metal shroud as shown in the below pic:
Once removed you will see the evap coil and if you look closely at the pic below of it you can see all the dirt/lint at the very top which you need to clean. You can also see that round "thing" attached near the bottom and that's the anti freeze sensor to protect against frost on the evap coil. The side you are looking at is the "intake side" with the area below open and where the air from inside the trailer is drawn up thru the interior filter. The other side is the sealed portion of the "plenum area" that starts the internal duct work for the A/C. If you look very closely you can just see a white protusion at the very bottom right of the evap coil and that is the drain outlet. There is one on each side of the A/C and there is a tray hidden directly under the evap coil where condensation collect and then drains out and onto the roof.
Here is a pic of one of the "drain outlets) and you can see the open area just below it where the water drains from the Lower A/C pain out onto the roof. You need to make sure those "trays are clear and you should be able to use something like a turkey baster to try and fill the pan below the evap and see if that drains out thru those trays.
Larry