KC,
I would almost bet that if you find the air drain valves you could back track and find the tanks, all coaches I've seen have tank air drain valves out where you can get to them. On my PT they are to the right of the generator attached to a frame member in the front and to the left of the motor on the rear.
Tell the tech to get out of the pit and start looking in the basement compartments and under the edges of the coach for the drains. If I were a so called tech I don't think I would complain to the customer that I was unable to find two air tanks that size.