That's my setup too, and my 2nd A/C still shuts off in low speed without obtaining desired temperature. Dealer still cannot figure it out and get it fixed, but they would not do anything about the low pressure going through certain ducts.
Here is a long list of what I did to fix the problem:
First, take off covers off A/C from the loft. Get some HVAC foil duct tape. Take out the divider in the A/C and start re-taping the open gaps where they connected the roof ductwork to the housing of the A/C. They typically do a lousy job at the factory. Once done with that, reinstall divider and re-tape around the joints of the divider. This will help from the return air sucking the supply air in the A/C itself, giving more output to grilles.
Next, take some aluminum foil and take off the grille adjacent to the A/C (directly above the feet when someone is lying down). Cover the entire grill with the tin foil and reinstall grill. If you want more air movement out of that grille later on, poke a couple holes in it using a screwdriver.
Next, unscrew the wood cover off the void at the back of the loft area. There is a duct that runs from the ceiling down to the bedroom ceiling. Cut the thin wood around duct to get hands into the ceiling. You will likely need to re-tape the transition from the supply duct to the flex duct. Get another piece of thin wood and re-seal the ceiling, and get some window/door foam to seal the penetration. Don't use regular great stuff as that has a lot of pressure when expanding. On my camper, I had to do this because too much air was flowing into the roof cavity instead of out of the vents.
Next, take off the ceiling grille in the bedroom and pull down the excess flex duct. Trim off excess but leave enough to reconnect and reinstall grill. What you are trying to accomplish is have the least amount of bends in the flex duct to maximize CFM's out of the grilles.
Lastly, take off ceiling vent by the door and do the same thing with the flex duct. On my unit, I also took tin foil and covered half of the grill. That increased the flow out of the ceiling grill by the back window. The bottom of the door for the bedroom and the stairway acts as the warm air return for the upper A/C, and since this blows by the thermostat located by the rear door, it should function and cycle better.
I would expect all this work to take about a day to do, but it really helped in my 4092BFL. Sorry for the long post.
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