In Greenville, we stayed at the Moosehead Family Campground, in a 45' motorhome. The location was great, but our camp site was not very big or level.
We plan to go to Greenville again - we love Moosehead Lake, but will be staying at the Balsam Woods campground. While their we went up the west side of the lake to the Birches resort. They have a small number of camp sites that look nice. We rented a Pontoon boat and took it around the southern part of the lake. The grandkids loved it. They also have scenic flights out of the Birches.
One thing you don't want to miss is the Golden Road, which goes from Greenville over to the Baxter State park area. It's a long and very scenic dirt logging road. You might see a moose or two.
The Balsam Woods campground has some outfitters that can take you on various tours.