Sept, too early for leave peeping but the kids are back in school so it's still the best time to visit.
Some CG's close right after Labor Day, most after Columbus day. Earlier the farther north you go.
Be ready for rain.
Skeeters are gone if we get an early frost.
In northern Me in Sept we usually take outer wear we can layer, gets fairly cool at night.
Many stores in the summer tourist areas will be closed or closing as are some restaurants. You can find deals though as they want to clear out the shelves some.