When traveling, we always reserved by phone, day to day, looking forward on the day's travel schedule. It was either "not a chance," "we've reserved for you," or "no reservations necessary" depending on time of year and other conditions.
For commercial campgrounds, longer stays, we have usually reserved by phone, although more have started to come online with a WWW presence (particularly KOA).
For USACE campgrounds and other sites managed by reservations.gov, we use reservations.gov, recognizing that not all sites in all campgrounds can be reserved. This because the web site is now the only access, if we show up at the gate (and someone is actually there) they need to go the reservations system to find out what is going on in their own campground.
