GPS systems come with millions of built-in POI's (Points of Interest), but invariably they do not always include POI's that are of particular interest to you. So most systems allow you to import custom POI lists that cover your interests, such as Corps of Engineer campgrounds, Wal-Mart locations, state parks, grocery stores, museums, etc. The trick is to find or make the POI lists that you want and then import into your GPS system.
You can find hundreds, if not thousands of other POI lists on the internet, free to download. Here are a few sources that offer campground lists:
The POI Factory - this is the Granddaddy of 'em all !
Jon Vermilye's Maps, GPS and POI Page - Dozens of informative links
Now you've downloaded your POI lists, you may have to convert them to a format recognizable by your GPS system. Here are some places to find out how to do that:
PoiEdit - manages all of your POI files from your desktop PC and saves/loads them to/from your Mobile Device.
GPS Babel - converts waypoints, tracks, and routes between popular GPS receivers and mapping programs
GPS Visualizer - converts various file formats on-line