OK, now thats a little on the strange side.
A few questions if I may. Is this a new problem , or are you new to the coach ? If you run the A/C on recirc, do you get the same issue ? As the fan speed drops do you see any change in the voltage gauge ? When the fan is running does it operate smoothly; no vibration/noise.?
Possible that air -flow around the coach at speed is pulling air away from the heater fresh air inlet, if the problem is not apparent in recirc, then that could be it.