Hermit Island is a fantastic place to camp. Been there a few times. But the reality is that getting reservations there will be iffy if at all. Usually they book our a year in advance.
I'm going to recommend here: Recompense Campground/Wolfs Neck Farm. You can go here:
https://foursquare.com/v/recompence-shore-campground-at-wolfes-neck-farm/4e054531e4cdefcff6ce6c7f
You've got to know that the crowds of tourists become pretty heavy now until mid September. If you want to see Cadilac Mountain, you'll need reservations so call to make them as soon as you can.
Good luck. My wife and I have tent camped up here in Maine for decades and, when we retired, we decided to finally move here and enjoy Maine year-round. We have never, for one second, regretted that decision after three years.
Rome