Maine is a large state and bills itself, justly so, as Vacation Land. So it’s quite busy all summer long. Most of the CGs open in early may and are closed in the first week of October.
I’m from Maine, born and raised. In my early years my father had a private pilot license and we flew all around ME & NH exploring the possibilities for remote fishing areas.
It’s my experience as an RV traveler that you cannot get more than a glimpse of Maine in 10-14 days. If you want to explore the coast of Maine you can do most of it from Kittery to Portland in a couple of weeks. Going north to Bangor and then down the Penobscot river and then up to Acadia/Bar Harbor will take another 10-14 days. From Kittery up to Fort Kent is about 375 miles. It’s very scenic with lots of farming country side attractions.
During our RV traveling years we visited Maine numerous times. Being from southern coastal Maine we seldom ventures beyond Portland. We also stayed away from the popular main stream CGs along US1. By going inland 12 – 15 miles you can find great family owned CGs that are quiet and family oriented with great amenities away from the hustle and bustle of the vacationers but still very close to all costal attractions most summertime visitors seek.
My first recommendation is Beaver Dam CG in Berwick ME – very close to my home town of North Berwick.
https://beaverdamcampground.com/
My next recommendation is up the road to Alfred Maine, just a short drive to Sanford where I went to high school.
https://walnutgrovecampground.net/
Up north we like the huge sites at Timberland Acres RV Park and it’s less than a mile to a super Wal-Mart.
https://www.timberlandacresrvpark.com/