Dometric ccc2 thermostat

If the temperatures are far apart, it will only run on hi fan. Adjust the Tstat temperature only a couple degrees above the room temperature since you are currently trying to heat your rig. Did this slow the fan down? If not, Try a system reset...with the Tstst turned Off, press the zone and mode buttons at the same time. IniT should display and then less the on/off button. Does it now control the fan speed? If not then the dip switches on the control board are not set properly.
Where in B’ville are you?