I go under my truck and pull the cables on the release valves on my tanks every once in awhile. If you get under there somewhere up around the transmission you should see the air tanks. They have small valves that have a cable attached to them so you can pull the cable and let the water out. Don't worry about the seats of the axle. The water condensation is most in the bottom of the tanks. The lines that run to the axle and seats come out of the top of the tanks so the moisture should stay in the bottom of the tanks.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio