We stayed 5 nights at Bar Harbor Campground last September. We arrived on the 18th, a Tuesday. First and last nights FHU for $44 and other 3 nights no hookup for $34. We paid by personal check. We drove up from Camden, ME and probably arrived early afternoon. No issue with getting a site. The office gave us a wooden "occupied" sign and told us to drive around to select a site, leave the sign on the picnic table and come back to register.
If you want to visit the town of Bar Harbor and don't have a toad, here's how we have always found parking for our Class C. From Bar Harbor Campground (assuming you stay there) take route 3 and turn left at Bar Harbor St. Go several blocks to just past Catholic Church and turn right onto Ledgelawn Ave. Go several blocks and turn left onto Park St. Go past the park with ball fields and turn right onto Main St. There is a wide shoulder alongside the park used for RV parking.