duo therm thermostat problem

Hello, I have a 34' 1999 coachmen 5th wheel. It has a duo therm by domestic thermostat. Model # 3107612.008. I am having a problem with the a/c not kicking on the fan when in auto mode. The compressor turns on but not the fan. You have to manually turn on the fan. Any idea what could be wrong? Do you need any more info? Thanks!!!