Do you have the Dometic Digital thermostat?
If so, then I agree with what dunnnc
had to say.
Here's some info that might help you.
Use the On/Off Mode button to select FAN. Then use the +/- button to select either Hi, Lo or Auto.
When using the AC and the fan is in the Hi or Lo setting it will run continuously while the compressor will cycle on and off.
In the Auto mode the fan will cycle on and off as well.
If the fan is set on Hi or Lo while using the furnace the AC fan will run continuously, so be sure to set the fan on Auto when using the furnace.
You can, however, leave the AC fan running while using the furnace to help with condensation problems.
After setting the fan, Use the On/Off Mode button to select COOL for AC use. Use the +/- button to select the thermostat temperature setting.
Use the same procedures for the Furnace.
When using the AC, in the Auto mode the fan will run on high when the room temperature is 5º or higher than the thermostat setting. When the room temperature drops to within 4º or less than the thermostat setting, the fan will stop for a few seconds the start again in the Lo mode.
Hope this helps.