That has to be a voltage fluctuation problem. Maybe your converter/charger is switching on/off, shifting your power back and forth between battery (around 12,6v) and converter power (usually around 13,6v). That's enough to change the fan speed.
The converter might switch off if it is overheating from inadequate airflow (it has it s own fan). Or maybe its just crapping out.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL