We have camped extensively in Maine along the entire coast. There are large expanses of coastline that are not inhabited but no where that I'm familiar with will be open to pulling through an RV and just stopping for the night. Your best bet would be to explore this website, CampMaine, that will guide you to what's available along the entire coast. There are some spectacular places to camp. Suffice it to say, however, that the three summer months are tough:
https://campmaine.com/
We've had excellent success in September and October especially in Downeast Maine. Again, that website above will help you.
If you are looking for some no reservation pullin places, look no further than the kancamagus highway which spans New Hampshire. There are a lot of first-come-first served camp areas there but, again, it'll be busy in the summer months. Acadia is either the most visited or second most visited national park in the entire country! It gets incredibly busy there and frankly, we stay away. Look much further North like Cobscook Bay. There you'll have some peace and quiet and enjoy the ocean at your feet.
Rome