Jeff, Where Greg describes the air dryer location is correct for earlier coaches, your Angola possibly having it there.
On the 97 it is likely high up on the frame ahead of the drive axle, passenger side. If you see evidence of oil on the coach around where the dryer "spits" you can bet it is far beyond the element's life limit. Failure to keep the dryer serviced could lead to moisture getting into the bus air system, and the likely impact could be corrosion from the inside of all valves and moving parts. At worst the system could be filled with water and you could end up with the Norgren suspension valves not functioning at all.