We only camped there once at the end of September right before closing. We had a Volkswagen EuroVan so we used the heater inside, which consequently drained our battery! There was frost on the ground in the morning when we got around to asking the camp host for a jump start, so it must have been below freezing. If I had to guess, I would say lows between 25-35 F.
It gets cold, but it is beautiful there in September.
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