I've seen some discussion here about frustration with these in-ceiling vents, and air conditioning effectiveness and thought I would share my 'improvement'.
I was not convinced that I was getting good circulation with my air conditioning vents, so I dug in. Some others here, including Lou have shared some things regarding this system, particularly the proper installation of the vent plumbing.
When I visited the Prime Time assembly line for the Crusader and Sanibel 5th wheels, I noted how the in-roof ducting was plumbed. It is actually a nicely thought out design with insulated ducting inside the 'attic'.
The first thing is the removal of the vent covers for which there has been several queries here as to how to do this. It's still not easy, but for mine (and likely many others with this type of vent) there are 3 tabs that need to be pulled out laterally so the cover is released. I used a flat bladed screw driver to do this. Just get one tab off and the other two are easy.
The mating part mounted to the ceiling fits loosely into a corresponding round hole cut into the duct which is some type of 1/4" hard foam. The factory uses aluminum tape to seal the duct to the vent body. Unfortunately, mine all looked like a 1 year old had done the job. There were bunches of sticky tape in the vent along with lots of remaining gaps between the vent body and ductwork. It might have been better had they not put tape on at all. This causes a lot of barriers for efficient air flow, plus the gaps allowed air to get into the attic.
I removed all of the factory installed tape and replaced it with new for a smooth, tight installation. You can easily feel inside the duct for gaps when taping.
Replacing the vent covers is as easy as snapping them back on.
I checked before and after air flow, and noticed a VAST improvement.
The only thing I haven't checked yet is the cold air return vents, so that will be another future project, but I know that all the cold air the unit is producing is getting into the interior now. This is an easy DIY project that anyone can do.
I'm pretty sure my rig isn't unique, so there are likely many others with questionable installations too.
I hope this is helpful to all here.