Usually, we do not make reservations and that has seldom been a problem. Sometimes, when we see that we will not arrive at our intended destination until after the office will likely be closed, we call ahead. Usually, we get to talk with a live person and find that reassuring. Oftentimes, the person will give instructions for late check-in. Sometimes she will promise to stay late until we arrive. Calling ahead like this is generally preferable to just showing up and then looking for a site, especially if we will arrive after dark.
On one occasion when we did not call ahead, we wished we had. When we arrived at the RV park, which was some distance from the highway, we discovered that they were no longer in business.