What kind of AC unit do you have. I have the standard roof mounted Coleman AC. The air coming out of the vents was minimal to say the least. I took the inside cover off and found that much of the air coming from the AC was going right back up the return side due to poor installation. I also found the holes that were cut for the ducts were smaller than they could have been and enlarged them. And lastly, I noticed a lot of air escaping around the inside cover, corrected it.
Your situation may be different of course, but first check to see if you are losing air.