I happened to watch a video on YouTube posted by Russell of 2 Trail Hikers where he discovered the divider between the A/C output and return was not properly sealed causing alot of the cold air being sucked right back into the return from the un-taped sides of the divider.Thor only put one piece of tape across the top of the divider. This also causes a drop in air flow to all the ceiling vents throughout the coach.
I had very low flow to three ceiling vents so I dropped the A/C cover and sure enough mine looked just like Russell's so I sealed the divider across the top and both sides. BAM!!! The increase in air flow was fantastic, now the three vents that hardly had any flow were now blowing at least 70% 80% better. I recommend anyone with the Coleman Mach A/C unit to drop the cover and check the divider. Seems to be a very common problem when Thor installs the A/C unit.
It's an easy 20 minute fix requiring only a screwdriver an a roll of aluminum tape. Just remove the air filter screens and the six screws holding the cover to the ceiling, realign the insulated divider and tape around both sides. All credit goes to Russell!!!
Here is the link to the video: