how to remove the trailer and parking air supply knobs

Might be an allen head set screw on the side of the knob that needs to come out.

 

Threaded on. Righty tighty, lefty loosey.

 

I don't know what to tell you. Mine are just threaded on.

 

Look closer.....Its a teency weency allen screw usually sits at the base of the knob

 

If there is a set screw in the shaft it'd be fairly small. Might not be all that easy to notice. Some one may have glued them on at some point too.

 

The set screw is sometimes on the bottom side of the shaft, might need to get on the floor and look up to see it