Raise engine access door, look to your upper right, there is a metal panel. Remove about 6 screws and the panel can be removed exposing most all of the duct. Actually, there is a photo and explanation in the most recent issue of FMCA mag...one of the reader-provider tech tips.
I just ran a test in hot and sunny Oklahoma. I have my motorhome setting in my driveway. I had my thermostat set at 75. When it got up to 97 I went outside and into the motorhome to see what the inside temp was. It was holding at 75. I then turned down the thermostat to 72 and went back into the house. Later the outside temp got up to 101. I went out to check the temp inside the motorhome. It was at 73.
This test aligns its self with past experience's I have had in hot weather. I would suggest that if you are not having the same results there is something wrong with your unit, not the central AC in general. Like others have suggested I would check to see if you are not leaking cool air to the outside through the duct under the back cap. I found a leak in the joint between the box that connects to the AC and the duct which leads up to the top of the motorhome. When I fixed that I could discern a noticeable difference in the amount of air coming through the outlets. I would make sure there are no obstructions to the intake ductwork. I use the highest quality 3M AC filters available. The higher the quality, the more air can be passed through the filter. Make sure there are no openings to the outside. Like the seals around the openings to the motor compartment. I found that the opening where the steering column goes through the floor was not sealed. If none of these suggestions work for you then make sure your second compressor is actually functioning. With the second compressor running you have the equivalent of 2-Tons of AC. That should be more than enough to cool such a small space.
I did not post until now because I could not run this test until now. I hope this helps.
I spoke with winnebago and the 20 degree difference is correct...but make sure your second compressor is coming on. Also, Is the A/C designed to run on one compressor and only a 20 amp or 15 amp plug? I thought you needed at least 30 amp cord plugged in?
It is iffy to run on a 20 amp circuit but can be done. Be sure to set your Energy Management System to 20 amps. If the circuit is a 15 amp circuit I doubt you can do it.