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Hello, my name is Joe
Typically, there is a blower speed resistor that enables the different fan speeds. Often the resistor will fail.
Typically the blower resistors are located in the front outside of the AC/heater plenum box.
See the image for a typical blower resistor location.
Thanks for the update,
Some a/c systems use a relay to provide the blower with a high speed circuit.
If yours has a relay, it will certainly be mounted somewhere outside of the Workhorse / chassis fuse box.
Does your coach happen to have a relay screwed to the outside of the heater plenum like the one in the picture?
Can you see a name or brand on the a/c plenum? Evans tempcon , or Denso are popular a/c system suppliers to the motorhome coach builders.
Just checking in to see if you need any clarification on something which I stated above. Please remember to rate my service, so I can receive credit for my help. -Joe