The wiring harness from Freightliner, to the manufacturer, comes loose on top of an empty frame , the coach manufacturer builds the dash and depending on who did the dash install, the chime could be anywhere within a three foot radius of the fire wall main harness connector.
I know not what you wanted to hear.
. It took me 3/4 hour to find mine , much easier with the dash cluster loose and tilted forward, the good news is once you find it it's easy to re-locate for next time .
I don't have a picture of the location of the sensor , I have seen mine , but if you follow the air lines from top pots on the drivers side rear air brake you will find it as described on the frame rail.