Air nipple should be located on the front bulkhead where there is a manifold for air lines. This is the only approved location to tap into the air system. Yours may be in the front compartment located below the driver, forward of the wheelwell.
On the CLIPPERS, it is located on the drivers side front bulkhead accessed via the front hood/door. Upper drivers side on this unit behind the front access door.
If you also have a front access door and not a genset up front, yours should be in the front also.