For the camp sites and the rental cabin you do
We stayed in site 21 in the north loop over Labor Day weekend. It is one of the few, or only?, reservable sites with electric and water in the north loop. Even over the holiday, there were still plenty of available spots in that section. Keep in mind it's more suited to tent camping or if you have a small trailer (we did). And the water hose is really close to the host site next door, not technically at the campsite. The site was conveniently across the road from the restrooms. Showers are about a 5 minute walk near the beach /picnic area.
South loop campsites need a reservation. The north loop is first come first served except for one site that has power and water.
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