This is one of those items that when you need it, you need it.  It’s not even so much about losing the A/C on super-hot days, as it is about losing all the food in the fridge.  Having to re-attach a vehicle in order to charge batteries in the event a few hours turns into a few days.  If you are full timing, this is a must have.  You don’t need the biggest one, but to run the basics and keep at least one A/C running, is a life saver!