Take the route up I-95 to Portland - Exit 44 [Rt. I-295] follow it to its junction with I-95 south of Augusta. Continue North on I-95 to Exit 182A for I-395 which will take you to Rte 1A South to Ellsworth. In Ellsworth continue straight on Rts. 1 and 3. Follow Rt. 3 to the causeway onto Mt. Desert Island. At the traffic light, stay to the right which is Rt. 102. Follow Rt. 102 for about 15 minutes to Southwest Harbor. After going through the village, turn left on Rt.102A for exactly 5 miles to Bass Harbor Campground.
Or, you can take this route which is almost the same length, but gets you off the Interstate. There is a new exit  at Augusta with a new bridge over the Kennebec River. (No more Augusta traffic circles!) Rte. 3 is a good road as is Rte. 1. There may be some traffic slowdowns.
Take Rte. I-95 north to Portland - Exit 44 [Rt. I-295] follow it to Rt I-95 and the Augusta Rt 3 exit. [exit 113] Follow Rt. 3 signs through Belfast - Searsport - Bucksport - Ellsworth. In Ellsworth take a right at the 2nd traffic light. [Rt 3] Follow Rt 3 to the causeway onto Mt. Desert Island. At the traffic light, stay to the right which is Rt 102. Follow Rt 102 for about 15 minutes to Southwest Harbor. After going through the village, turn left on Rt 102A for exactly 5 miles.
Welcome to Bass Harbor Campground!
(You could take US Rt. 1 all the way from either Portland or Freeport to Ellsworth if you would like to see the picturesque Maine Coast. Depending upon the traffic, this route could take from 5 to 8 hours!)