Yes some time it is needed, chassis Air will cool the cockpit not the whole RV,
Yes you are burning gas
No it does not change "Actual" fuel mileage of the drive engine, it just LOOKS that way because of using gas for another purpose
Yes you use more gas , but that was used by the generator not the drive engine, if anything drive engine load was very slightly reduced , because the chassis AIR did not have to work as hard at cooling,
Very slightly equals maybe a few tenths of percent less load
If I got worried about the extra fuel use,
I would turn off the chassis Air, and use the generator & rooftop air , it would be warmer up front, but the MH would be cooler