How are "campsite" and "campground" used in American English?
I've done a lot of camping over the years: backpacking, car camping, off-highway vehicles (Land Rovers), etc.
A campsite is where you camp. The place where you pitch your tent or park your vehicle. In a more developed area, it's likely to have a picnic table and fire ring and be fairly well defined.
A campground, on the other hand, is a group of such sites. A single campground may have a few campsites or it may have hundreds. It could also be used to refer to a group of campsites within a single park or area.
Hope this helps!