Exhaust fan for AC

I have a 2018 also. Yes run the exhaust fan when you run AC. The fans direct air down out the bottom vent under the T@G. You can also run anytime just to exchange some air in the T@G. The air is pulled from the two chrome vents on the sides back upper of your rig. If you forget and do not run the fans when the AC is on, it not a major issue. 
 

The purpose of the ac fan is to expel heat build-up in the area behind the AC since there's no window for the back of the ac to stick out of.  They pull air from the cowbell like vents on the side of the T@G and push the heat out the bottom.  I turn that ac fan on before I turn my ac on so that I can be sure the fan is running and doing its' job.  Side note:  Those cowbells on the side of the T@G can be a source of dust while traveling in dusty areas.  We put sponges in ours while traveling to keep dust out.   And, of course, added removing them to the setup check list.