I was out at Quartszite RV show weekend before last and stopped by a booth that sells a diverter that installs in the plenum area of the AC unit and smooths out/directs the flow to the ducts.
https://rvairflow.com/ It did have a much higher air flow, I think the efficiency increase was definite over how the air normally blows into the plenum and from that turbulent area has to find its way to ducts. They had systems set up with air flow meters. I probably would have bought one but they are about $200 and it's basically a piece of dense closed cell foam. I know, it's the development that costs, but I just couldn't plop down $200 for that. $50-$75 maybe.
Here's an install video:
https://youtu.be/Znb6Ji4kAR4