We were there for 2 weeks 2 years ago in July. It was 95 degrees and sunny. Very odd weather for New England especially in Maine. We had packed our sweat shirts and long pants for evenings around the campfire but instead sat in tank tops and shirts in the A/C of the RV! Had to cut some of our hiking trips short and just spent more time at the beach. So with New England you can never predict the weather.
Crowds are not too bad. The free bus is great to get around. When you get there just pick up a cruise ship schedule and avoid downtown Bar Harbor the days the ships dock. Down town get very crowded then during the afternoons when all the passengers come ashore.