I agree with dougrainer that you need a larger A/C unit, he is also right about the differential. But I am pretty sure that you don't believe that the A/C unit was designed to run slow air speed, rather that something is causing slow air speed. That something should be causing the evaporator to freeze up. I have never seen a Coleman A/C freeze up when the ducts run slow - the ducts air will run slowly because they are trying to push a large amount of air through a small hole into the large duct to quiet things down. The air passing over the evaporator coils will be just as great either way. To prove it, measure the size of the hole that the air goes through to enter the duct work - I expect it is something like 4" X 6" and then the ducts are 3" x 10" that means the air passing through the 24 sq in will speed up to get to the larger volume 30 sq in x 20 ft or more. And those ducts are both directions so they are times two compared to that little hole the air is forced to enter the fan through.
