Vehicle length in National Park Campgrounds

There is absolutely NO standard length in National Parks. Each individual campground will/may have it's own limitations which are based on the conditions in the campground and how easily THEY believe it can be navigated plus the obvious limitation imposed by the actual length of drive ways at individual sites. Go to Recreation.gov and look at the PARTICULAR site you're interested in and it should give you any maximums that apply to that site.
Good luck / Skip