AC unit for camper intermittent?

I have a 97 Terry Travel Trailer when I turn on the thermostat to Auto Cool 90% of the time it will not work however if I turn it on On Cool 90% of the time it will work however the fan will stay on 100% of the time and the compressor will kick on and off as required. The question is does this sound like a... show more I have a 97 Terry Travel Trailer when I turn on the thermostat to Auto Cool 90% of the time it will not work however if I turn it on On Cool 90% of the time it will work however the fan will stay on 100% of the time and the compressor will kick on and off as required. The question is does this sound like a thermostat problem or a Freon problem and if it is Freon what type of Freon does something of that age require? Thank you for any help you can give.
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