What you describe "MAY" be normal operation. The fan is a 2 speed fan, HI is 1300 RPM and LOW is 1100 RPM. It's difficult with the noise of the air conditioner to actually "discern the difference in fan speed" unless you're very focused on listening. To the average person, both speeds "sound the same"...
That said, the thermostat/air conditioner, when the fan is set to AUTO, will vary the fan speed depending on the difference between the set temperature and the sensed temperature on the thermostat. When the "special feature" is working properly, the fan will stop, compressor will continue to run, and the fan will then turn back on at the different speed. It may cycle on/off multiple times if the trailer temperature goes down with fan on high, and as it gets to the "set point limit" change speed, the temperature starts climbing and it recycles again. It may do this all day or it may not, depending on the outside temperature, how much heat gain is in the trailer and multiple other factors.
Here's the statement from the Dometic thermostat manual:
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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