I had one RV that had exceptionally poor airflow and I found 6" round stepdown diffusers and that did the trick. The stepdowns have more free area/less restriction and farther throw. I do the same thing in offices, or wherever, when airflow/distribution is a problem. Flat bottom diffusers are the most common and most of them are junk but I see stepdown diffusers in well designed buildings that didn't have such a tight budget. You can put a balancing damper or shut-off damper above the diffuser but they are usually too thick for an RV. I used to be able to buy pull-chain dampers for the top of the diffusers and people could play with them at will. They fixed 90% of the seasonal air volume problems in offices. Craig
