Hard to say if it is "really needed" - We have it because my wife insisted on it. Here in the (mostly) high desert and mountains of the western US I am very comfortable, even on hot nights, running nothing more than the Fantastic Fan (12VDC) in the roof. That said, we spent a year in the Florida panhandle and the AC was nice on sticky, humid nights.
We run a ClimateRight 5000BTU unit which is both a heater (more useful to me) and air conditioner. It is a 120VAC unit so I have a Honda EU2000 genset installed as well. Since the genset is mounted to the frame you can feel it running so, when climate control is desired/required, we run it and the air con / heater to get the temp where desired then shut it off when retiring for the night. Everything, along with the battery, is installed in the tongue box of a SoCal Teardrops Krawler 459, i.e., not a very large trailer.