One other option is to stay on Rt2 all the way to Bangor. It will dump you right onto I395. The section of Rt2 between Newport and Bangor has been resurfaced and is now in good shape. And in my opinion is a more relaxing drive.
Iwas looking for the info that dhayes gave me. Two lane is fine, as long as the the road will allow for my little coach. Are there any low bridges or super hard high hills to climb?
US 2 is the best route. Although it is mostly 2 lane, it is by far the most scenic. Once you hit Newport, Maine, take Interstate 95 to the outskirts of Bangor. There you pick up Interstate 395 across the Penobscot and around Brewer. When 395 ends, follow Highway 1-a into Ellswworth and just go straight. You'll start seeing the hills of Acadia about 8 miles out of downtown Ellsworth. Enjoy Acadia. It's a great park.