How do campgrounds charge you?

Charges do vary pretty widely, to answer that question. The variation is a combination of geographic differences (some areas of the country are more expensive than others, and campgrounds in cities or near popular destinations may be more expensive), amenities included or available, site size, the general "niceness" of the campground, the time of year, whether it's privately or publicly owned, etc.

Some campgrounds (and some sites at many campgrounds) may be physically only able to accommodate modestly sized RVs.