best camping in May in or near Acadia National park

Acadia National Park only has two front-country campgrounds, Blackwoods and Seawall. The latter only opens in late May, so this may be a factor given your trip timing. Reservations are usually recommended for Blackwoods, though you should be OK, I think, if you're traveling in the early to middle part of the month as it's still before the main season (it'll probably get busy around Memorial Day at the end of the month). There is also a five-site primitive campground on Isle au Haut called Duck Harbor Campground. This island is only accessible by mailboat, so a more adventurous option! Have a great trip.