I very rarely use a campground or RV park that allows or encourages reservations.
In fact, right off hand, the only one I can think of is the campground where the Glacier/Waterton Hamfest is held. Advance reservations are a must there, if you want a site with hookups. If you will be happy dry camping in the meadow or under the trees, reservations are optional, but you better get the early on Thursday to get a good spot! All the shady spots will be gone by late Thursday afternoon. No, you can not reserve your favorite shady spot. First come, first served!