The symptom: the dashboard fan switch only seems to function on HIGH. All other switch positions have no effect.
The cause: fried heat fuse in the blower resistor assembly.
The cure: either order a new $40 assembly or spend $3-5 on a new fuse.
In Monaco products, the resistor is carefully concealed at the top of the dash AC plenum, accessed by opening the nose of the coach. In my case, I had to cut through the underside of the dash (very carefully, to avoid slicing wires) to provide enough space to remove the two mounting screws and remove the resistor assembly.