According to the National Park Service the overall length is 55' so if you're towing make sure you are within the limits. That's why we can't go again as we are right at 60' with our toad. Years ago we did the whole parkway in a 29' Class C pulling a car and were fine, but not now. Would love to do it again but not taking the chance on getting busted for over length.
"Recreational vehicles are a popular way to see the parkway. The length restriction for RVs is 55 feet, including a tow vehicle, and the height restriction is 14 feet."