Some possibilities for you: Penobscot Marine Museum in Searsport is small but good. Maine Maritime Museum in Bath (as mentioned before) is wonderful and just down river from Bath Iron Works which has been building ships there since 1884, currently the best frigates for our Navy. Pemaquid Point in Bristol has a great lighthouse on one of the most trecherous spots in that part of the coast. One of my 11X Great Grandfathers was shipwrecked there in 1635 on the Angel Gabriel. He and one son came to the new world to get things set up before sending for his wife and other children. As a result of the shipwreck, both John and his wife were so traumatized neither would ever set foot on a ship again, and never saw each other. Google "John Bailey Angel Gabriel" for a good discussion.
South of Thomaston is Port Clyde with the Marshall Point Light showing the way to harbor. Well worth a visit. In South Thomaston we stay at the Lobster Buoy Campground, which is (shall we say) rustic, at best, but nice, quiet and right on the ocean. In Kennebunkport you might consider Sandy Pines Campground which is relatively new, great shower arrangements, and glamping. You're better off approaching from the Biddeford side rather than the Kennebunkport side as getting through K'port with a rig would be a task.
Wassamki Springs Camp Grouond in Scarborough might be a good place to explore Portland. I've not camped there (nearly in our back yard), but visited many many (40+) years ago and thought it nice. Portland is well worth a visit.
If I think of anything else of value, I'll post again.
fred.