Thanks for the ideas in this thread! After this very simple "mod" both my front and rear ACs on my 2023 New Aire seem to be putting out much more cold air into the coach now.
I went to a hobby store and picked up some 1" thick foam that I could tightly roll up like a newspaper and then it would easily re-expand. I rolled it up and then inserted it into the rearmost of the 6 front AC cold vents (closest to the bathroom), shoving the rolled up ends such they would each butt up against the sidewalls of the shared air duct.
Doing this basically divided that shared cold air duct into two: one for the main coach and a seperate one for the rear bedroom and bath.
Now when I run the rear AC more than half the air isn't instead going out the front vents of the coach. So much is going out the rear bedroom vents now when the rear AC is turned on I can not only feel the difference coming out, but even hear the air. No more questioning if it is even on or not. I also have more air coming out the front vents when the front AC is on as some of it isn't making its way to the bathroom and bedroom.
I think this hack works because the front AC sits directly between the 2 pairs of 3 cold vents up front (center of the two fancy vent covers) while the bedroom AC actually sits above the bathroom just a few feet behind those 6 front cold vents and yet what seems like miles from the rear 3 cold vents.
For better air flow I did leave the return vent alone, allowing it to suck air in from the easiest path given how many have added additional supply vent holes to try to solve this issue. That said one could do the exact same thing in that 6th supply vent to make the ACs truly seperated.
So far so good. I wish Newmar did this at the factory, but it is an easy hack to increase air flow and how much better the 2 AC units appear to work.
I have included photos. Please excuse the poor quality on the scope photos from inside the vent.
Best,
-Mark