We're real big on reservations. I'm not much on driving to various campgrounds and just seeing what is available, if anything. After a long day on the road it is nice to just check-in, drive back to the site, set up and relax.
If you can't reserve online, call ahead and find out whether your rig will fit or not. Also, what Dan said about checking park review sites. We also use an app called Allstays (they have a web site, as well). They keep a pretty comprehensive listing of campgrounds and note if they are big-rig friendly or not.