On the Freightliner XC chassis there are three cables that are looped sticking through the frame behind the front tires. You might have to turn the wheels to the right or left to see them. Should be two on the driver's side and one on the passenger side.
Use the awning pull rod to pull the loop of the cable and hold it for the air to blow out along with any water that has collected in the tank. Do them one at a time. If the air dryer is working correctly, you will get very little water if any blow out. You don't have to completely drain the tank of air, just long enough to blow out any water collected there.
I believe the manual says to do daily, but I usually do it about every 500 miles when traveling or at the end of a short trip.
Ron