Jerry,
Your air dryer should be just inboard of your battery compartment. There are many schools of thought on the proper service interval for our coaches. The system was designed for an 18 wheeler that averages 300,000 miles a year, so I do not subscribe to the service interval in the manual. I service the dryer if I ever get any water from my air tanks when I bleed them. If there is no water in the tanks, the dryer is functioning normally, so I leave it alone.
Gerald