As many have mentioned...it depends on your needs.
Here in the South...A/C is a must have for a good part of the year. If I lived in Minnesota...I would have different needs perhaps. I would winerize my camper and not use it for the winter months perhaps. And having good shade...maybe some fantastic fans are all I would need. Maybe a small Honda EU2000 would come in handy. You could run most things you need when away from a full hookup.
Going full time...if you travel a lot ...you will need a/c to sleep well at night...might want to stop at a Walmart along the way, or a Pilot truck stop...having a built-in generator where you can fire up the a/c and sleep comfortably would be choice during the hot months.
For the snowbirds going to mild dry climates for the winter....many seem to engineer some pretty outstanding solar systems. There is a great deal of satisfaction living off the grid when you want to...and without the noise and fuel requirements of a genset.
Living in FL we get some power outages from time to time with thunderstorms...again, if it's hot and you're stuck inside from the rain...the generator would be nice.
I don't have one now...but my next camper will more than likely include one.