The vents are not adjustable, they are just holes at the bottom of the freezer section that allows cold air to drop into the fridge section.
An RV fridge cannot be filled to the gills. This will prevent the air from flowing around the contents to keep them cold. It's very important that the doors remain closed as much as possible to keep the cold air inside. For some, they have purchased miniature fans to help circulate the air in the refrigerator.
Anyway this is how my RV freezer/fridge works. It takes a while to get it cooled down, but once it is cold, it works fine. Just minimize opening and closing the door. I haven't needed to purchase a circulation fan.
When in transit, I will run my refrigerator to allow it to get cold in advance. In this case, I will use the 12v/propane option while I'm driving. This can be dangerous if you don't know the precautions because not only will the propane supply be on, there is an occasional flame for the absorption process to work for cooling the fridge. YOU MUST remember to turn off the fridge at the refueling stations and when entering tunnels, or wherever flames or ignition can be dangerous.
Other questions:
How long are you waiting for it to cool? Is it cold, then stops being cold? Does it ever get cold in the fridge part? What are you using to tell what the temperature is inside the fridge? Are you opening the fridge doors too much?