You can here the fan that is all the way in the back of the coach?
(The radiator fan is on all the time I do think and is located behind the massive radiator. .
Actually I think the fan you are hearing is the A/C condenser fan that is located under the coach, center, just in front of the front tires. This will cycle on and off based on outside temp. It has a high pitch that drives me nuts.
With that said, diesel engines can move up and down in temp. They do not warm like gas engines. In fact, they really do not get hot at idle. (By hot I mean enough to produce heat at the vents.)