Being from Maine, I get the chance to go often to Acadia.
Schoodic Woods: Situated in Winter Harbor (about an hour from Acadia NP). They do have a free shuttle that will bring you to Acadia and Bar Harbor. Sites have water /Electricity but there are NO SEWER. The nicest campground of them all.
Bar Harbor Campground: No Reservation. (first come, first serve).....maybe not your best option. I think they don't take credit cards.
Smuggler's Den: Situated in Southwest Harbor. Good sites, full hook-ups and free shuttle to Acadia
Mt Desert Narrows: Nicely situated,but I think they are over priced. On top of your campsite fee, you have a resort fee !!!!!!!
Timberland campground: Located in Trenton....Free shuttle, full hookups. campground is not bad at all.