We were just looking for a site on the Oregon coast for spring break, a month out. Several private sites showed no vacancies. I tried adjusting the dates from single nights to the entire week with no luck on at least 2 different locations. Our friends with more experience told us to call, when I did I found out they had quite a few openings, some for the entire week!
I wonder if these parks are paying a commission to a third party reservation system and reserving a number of sites for call in's so they don't need to pay the commission.
Regardless, there's a good lesson, don't rely on online results, there are often unreserved availability to those who call.