Vehicle length in National Park Campgrounds

Stayed at Fishing Bridge in Yellowstone recently and staying at Grand Canyon if a few weeks. At those two its the RV length only

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

When I read the particular site details, most of the time they will mention the length of the site. If they would mention length and width, this whole issue would go away. The description should let us know whether we can fit lengthwise or if we can fit side to side disconnected. Sure sounds like a simple process to me. Don't just tell us what length trailer would fit.

Forest Service Ranger told me "any single vehicle" I take that to mean you will be OK for 35 feet or under.