So I tried restricting the A/C cold air output vents. I cut down the test plenum so that the output opening is 3" tall by 2" wide. I cut it down that much for another reason. When I push the A/C unit in to stow it there is only 2" of room in front of it. I had hoped that I could permanently attach the duct to the A/C. I did not want something I had to store separately and then attach at setup time.
So far the restricted plenum seems to work. I was able to get the sleeper cabin down to 70 degrees and 48% humidity. With the large plenum I could not get humidity below 64%. The compressor now runs considerably longer. The unit seems to cycle much closer to normal. I'm going to continue testing it tomorrow and leave it run most of the day. I want to make sure it doesn't ice up or do other weird stuff.
If it works out well I'll build something sturdy and attach it to the A/C unit for good.