Admittedly, I am certainly not the first person to do this.
Our big 2022 trip is going to take us 8000 miles and span some hot climates, so the single 15KBTU AC in our 2017 Reflection FW needs to work well. After some research and YouTube videos on airflow, I removed the face plate to our unit to see the crappy construction (first pic), turbulent distribution, and leaks. Using foam board and tape, I sealed everything, smoothed and constricted the airflow from the fan to the two ducts. I also sealed the isolation between cold side and return side. The project cost $17.81 in materials and increased measured airflow at the far end of the ducts (bedroom) from 5 to 11 mph. There are commercial solutions available for $170, but this felt worth the time. This does negate the option of blowing straight down out of the AC unit.
Subsequently, I also taped up the gaps in the AC vents, driving up the airspeed in the bedroom to 14 mph (280% improvement). I also replaced the fixed vents in the main living area with closeable versions ($44). With all the vents closed except the bedroom, airspeed in the bedroom is now 24 mph.
For reference, my inspiration: https://youtu.be/3Vhv_b-1zAk