For the holiday weekend that could be trouble. After that you might luck out most nights but would have to start looking around 2pm, by 6pm the "no vacancy" signs go up. It would be much less stressful to come up with a plan and stick to it, making reservations. Driving around looking for vacancies can waste a lot of time, and there are a lot of good places to stay that you won't even see just driving the main roads.
I don't know how "booking the day before" would help you if you don't know how far you're gonna get on day one. So I would just book everything in advance and be done with it. The distances aren't that far so it's not like you're not going to be able to get to your next destination. There aren't really a lot of places to stay last-minute along the coast between Portland and Acadia. Camden/Rockland is one. Maybe Boothbay Harbor or Damariscotta? There are usually roadside motel rooms available but why stay on the highway when you can stay by the ocean or in a downtown area?