The clutch assy freewheels until engaged so it may look like the compressor is turning. With someone inside the coach to turn on the A/C you should be able to see or hear the clutch engage when the A/C is turned on. If not then some permissive isn't being met, low Freon pressure is one item as mentioned.
If you can reach the leads on the compressor a voltage check with a meter will show if the permissives are being met. Use caution not to get fingers, hands or clothing caught in the belt.
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