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Most private campgrounds set their own standards and should be consulted before traveling as to their length restrictions. They vary much to widely to be answered in general.
National Park Campgrounds (State and Federal)
There are 25,700 campsites in 548 campgrounds at 77 areas of the U.S. National Park System. A broad overview of these campgrounds can be found here. It answers most of the questions anyone who has never camped in a National Park might ask, plus a link to the NPS campground reservation system.
The average maximum length of an RV permitted in a National Park campground is 27 feet, but some campsites can accommodate RVs up to 40 feet long.