Many printed campground directories are out of date by the time they are published. If you travel the interstates, you may find
The Next Exit useful. They also have an online version you can buy.
In my experience no single directory or website has a complete listing of campgrounds and RV parks. I use the following websites to find campgrounds along our route:
RV Park ReviewsPassport AmericaAllstaysRV ParkyWoodallsFree CampgroundsFree campsitesU.S. CampgroundsNational Forest Campground guideUltimate Public Campground ProjectCorps of Engineers CampgroundsI prefer the ones that are map based so you can easily find places along your route without knowing the name of a town. Some of them have data sets that you can download to a mapping program or GPS.
The POI Factory and
Discovery Owners Forum also have a lot of campground data sets and other Points of interest that can be downloaded to mapping programs or GPS.