Ceiling grill with vents are called 'chill grill'
The vents are directly connected to A/C unit discharge air plenum so air flow from vents is the coolest air......blows straight down into rig (and air flows out of plenum down thru ducts/registers in ceiling).
With chill grill vents open more fan blower noise and air flow noise can be heard.
Only time we use 'chill grill vents' is upon initial cool down when just setting up rv after travel day and rig is hot.
After that......they get closed due to noise. Makes my ears hurt :B