I would say your system is operating as it is designed. With the thermostat fan switch set to AUTO, the system fan will operate on HIGH speed until the thermostat "sensed temperature" is less than 4F from the "set temperature" and will switch to LOW speed. When the "sensed temperature" rises higher than 5F difference, the fan will switch back to HIGH speed and remain there until the "sensed temperature" falls back to less than 5F within the "set temperature". Once the inside air temp reaches the thermostat setting, the compressor will stop and the fan will continue to run for 1 minute. That "supposedly" clears the ducting of cold air, preventing or reducing the chances of condensation and mold inside the ducting.
It sounds like your system is operating normally, switching to LOW speed as the inside temperature gets within 5 degrees of the thermostat setting, and as the sun warms the trailer, the inside temp rises above that 5 degree difference and the fan switches back to HIGH speed. The cycling will continue to maintain the temperature difference, and on your system, it looks like the thermostat setting can't/won't be attained with the fan on LOW speed, thus it keeps cycling.
Completely normal operation.
It's all explained much better in your thermostat owner's manual. It's located in the "special features" section of the manual under "auto fan operation".....
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS