Driving from Boston to the Rangeley lakes region a couple weekends ago, we stopped for dinner at a place called Chuck Wagon, just off Route 4 in Livermore Falls, or maybe Livermore. It looked like the kind of place you get burgers, so we did, and weren't disappointed. Nothing fancy, but big and very juicy, with good fixings. Onion rings were good too. Avoid the salad, though.
They also had lobster rolls and such but we didn't try them.