The best approach to a GPS system is buy the GPS itself based on its specifications, etc., and then add the POI's that you want.
One of the most complete and accurate POI lists for public campgrounds is found at
The Ultimate US Public Campground Project. It includes all federal, state, county and local camping facilities, ranging from the traditional parks to lots of boondocking spots, over 19,000 of them. It does NOT include any commercial campgrounds.
The exact same information is also available as an
iOS app for iPhone, iPad and iPod. The app is really useful because it includes all sorts of filters so you can find just those campgrounds in which you are interested.
Ted H.