How long are you planning on staying at Acadia? I would be hesitant to recommend Blackwoods campground due to its being rustic and a tight fit, but a 36ft motorhome may be okay in there. NO hookups, dry camping. Great campground though!
Do have a towed vehicle? Because I don't know about how early the shuttle buses are... one pretty cool thing to do is to watch the sunrise from the top of Cadillac Mountain. From there, you will actually be the first people on the continental United States to see the sunrise.
Careful about the hiking trails in Acadia. The trail maps can seem deceiving -- what looks like a short, easy trail may actually be a difficult up and down climb. On the other hand, the "carriage roads" are really neat, especially if you have bicycles.
Don't forget to stop at the Jordan Pond House for popovers and coffee!