To find a campground using 'GPS', you need to find the longitude and lattitude of the campground, and enter it into the GPS. This is not really useful, since it will usually point you through fields and buildings, since 'GPS' is 'as the crow flies'.
A better solution is 'NS' or 'Navigation Systems'. This shows you the roads to use to get to a destination. So, to use it, you enter the address of the the campground, and then follow the directions. As stated, they are not always accurate, up to date, or practical, but usually they are pretty useful.
No matter what gadgets you use, a paper book listing campgrounds, prices and locations is a good thing to have. Or a computer hooked to the internet may be able to find some campgrounds in the area, and with a GPS input to the computer and the right software, tell you how to get there as well.
The BIG campground ads in the TrailerLife Campground Directory usually gives the address of the campground if they bought an ad. Or Google the campground on your computer and the campground Web site may give you the address. Plug it in to the GPS. If you don't have the address plug in the nearest City to where the campground is located and when you get close to the campground area use the GPS to locate nearby lodging/campgrounds under WHERE TO: Sometimes the campground you want is listed by name.
Another option is to search the web for POI's you can download into your GPS unit. I found hundreds of parks Private and State that I was able to add to my Garmin unit.
If I am just searching for parks I also like to use http://rvparkreviews.com/to get a general idea of peoples opinions and photos of the parks. Another thing I do which I have found extremely helpful...when I am going to a park/location that I have never been at before I try to get an image using Google Earth. Makes it real easy to findyour reservedsite at unfamiliar parks! Also helps locating the park entrances.