Campsites are on 1st come first served basis. They do not assign campsites in advance. When checking in you can ask if there us any cancellations next to you.
There is not an option on the reservation page, but like others have said, call the reservation number. Usually campsites are a first come first serve, so the earlier you get there the better. The best solution i have found is to start your stay during the week when there are less people in the park and the odds of you getting that many sites together is greater.
I would call and ask im sure they will try.
I recommend you call the reservation number and tell them the connecting sites you want and when. We have had good luck with that in other state parks.
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