Blackwoods campground in Maine (Arcadia National Park)

I'm not sure I understand what you're asking. They have sites that are 35 feet long, and some overhead something or other that's 11 feet off the ground.

My class C motorhome, for one example, is under 11 feet tall but just about exactly 32 feet bumper to bumper; with the bike rack on the back, I'd be pretty close to 35 feet long. Most travel trailers (not fifth wheels) are also comparatively low even if they are lengthy models.

Maybe the old bridges made out of field stone have something to do with it.

Maybe the old bridges made out of field stone have something to do with it.

Yes. Part of the park is only accessible by having to go under these bridges.

Per the Acadia NP web site-
Height Restrictions
Plan your route carefully. Many park roads feature historic bridges constructed with lower than conventional clearances. The shortest underpass on Park Loop Road is 11 feet 8 inches (3.56 m). Buses exceeding that height must exit Park Loop Road using Otter Cliff Road at Fabbri Picnic Area. If damage to a bridge overpass occurs, drivers will be cited and the park will seek restitution. Height restrictions are as follows:

Park Loop Road is closed to vehicles taller than 12 feet 2 inches (3.70 m) from Sieur de Monts to Sand Beach Entrance Station.
Park Loop Road is closed to vehicles taller than 12 feet (3.65 m) from Fabbri Picinic Area to Wildwood Stables.
Stanley Brook Road is closed to vehicles taller than 10 feet 4 inches (3.15 m).Fish House Road is closed to vehicles taller than 11 feet 6 inches (3.50 m).

re

Re read the driving directions> The entrance to Blackwoods campground is off HWY 3. There are no restrictions on HWY 3. The height restrictions are on the park loop RD. You can get to the campground. Just don't drive a tall RV on the park loop. Use your toad.

The entrance to Blackwoods Campground is off of State Highway 3, six miles south of Bar Harbor. There is no entrance to the campground from Acadia National Park's Park Loop Rd. The Park Loop Rd. has several low bridges and the majority of it is one way traffic; do not enter any entrances with a vehicle or equipment that is 10'4" or more.