Actually you want to call before 4pm for your best chances of getting someone. I call often and never seem to have an issue unless it is after 4pm.
Just keep trying, they are busy and only a few working at the office, call before 5 pm, call a few times consecutively. I had to call 4 or 5 times to book my campsite. If you will arrive before 5 pm just stop by the office and they will get you registered, it is not a busy Campground and certainly they will have something for you.
We stayed over the last weekend and sadly they never responded to our phone call or VM. Awesome location.
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