I-95 versus I-295 in Maine: there's nothing wrong with I-295. I-95 goes through Portland, but it's four lanes (two each way) throughout, BUT, you can avoid that by staying on I-95 to Exit 52 which is a connector north of Portland over to I-295. I-295 is shorter and has no tolls. I-295 joins up with I-95 in Gardiner just south of Augusta.
Once you hit Augusta, you can continue on (toll-free) I-95 to Bangor, then take US 1A to Ellsworth, or take US Route 3 across country to Belfast where you'll pick up US Route 1 and follow that along. Time wise Route 3 may be a bit shorter, and it's a nicer ride.
Our preference on Mount Desert Island is Bar Harbor KoA Woodside. It's cheaper than the Oceanside, has cable, and you won't be spending that much time in camp anyway. Get a site out back as the wooded sites near the front tend to be buggy. Sites at Oceanside which abut the water are not full service, but some of the others are. You pay dearly for it though.
As far as hiking is concerned, you'll have more than enough opportunity throughout Acadia NP.
You didn't say what time of year you'll be coming, but bring jackets. Even summers in Maine can be chilly of an evening, especially on the water.
fred.