As others have mentioned campground directories are a good start as are road trip planning computer programs and online sites. I do like the hear of someone's opinion on a particular RV park.
But there are a few things to consider and tips...for your 1st long trip out plan ahead to select the types of roads you wish to drive on & research the 1st couple of days campgrounds along that route. After that...try "winging" it day by day.
However we found it has always been difficult to guarantee we will make those certain stops on time. We found it's more "fun" to stop more often and on a moment's notice to see something between stops...that throws RV park reservations "under the bus"!
Online Campground directories and printed books are good but both may be out of date. Always call ahead. I found another way to "have a look"...using Google Earth look up the campground and really "look" around, especially helpful to see how easy it is to find the park. However,images may be at least 1 year old.
Call ahead, at certain times of year some areas are booked, in winter they may be shut down, etc.
Always arrive during daylight hours no matter how well you have researched the site. We've had issues that caused damage because of the dark.
Happy Trails...
