It could be anything. I would first check to see if the thermostat is actually working. Turning it slowly you should hear a click. Check your wiring connections to it. If your thermostat is working, it could be something with the starting components for your AC compressor. If your fan starts like you said, then you have power to the unit. Also, make sure your voltage is at least 110v-120v. The compressor and starting components are pretty sensitive to low voltage.
2015 Jayco 24FBS Elite Jay Flight
2013 Chevrolet Silverado 2500HD 4X4 6.0L 3.73 Axle
HD Trailering Package with Integrated Brake Controller
Equal-i-zer Hitch 10k/1000