My camper has a Dometic Duo Therm AC unit, with ducts in the ceiling. The unit itself works great, it blows very cold air.
However, there are a few things I've found that seem peculiar to me. It has a grate with a diverter that I can open for the "quick cool" option, and directly beside that is a larger grate where the filter goes.
None of that is a big deal, but this next part confuses the heck out of me, when compared to the conventional AC knowledge that I THINK I have.
The unit blows air OUT of the filtered side, and draws air IN through the side with the diverter. If I'm not mistaken, that "quick chill" and diverter slide mean that air should come OUT right there. When the diverter is open air should come out there all at once to quickly cool down the living area. Close the diverter and air is then forced through the ducts. As it is, when I close the diverter all that does is choke air flow INTO the AC unit.
It also perplexes me that the air coming OUT of the unit is what's being filtered. I will attach a few photos and videos showing what's going on.
I discovered all of this because I've never had a unit with ducts in the ceiling before, and was investigating why little to no air comes out of the ducts. Then, I was trying to figure out where the filter should go (it is sans filter right now). The more I looked, and the more I googled about this particular unit and where the filter goes and what the "quick chill" feature is, the less and less sense things made to me.
In one photo you can see the paper being sucked into the right side of the larger grate. It seems air is being inhaled on that right side and exhaled on the left side. It almost seems like air is rushing in from the bottom right there because it's heading into the ducting, not necessarily returned into the AC. Not really sure.