We tend to visit Yellowstone at least once a year, often showing friends or family around. The last couple years the "official" line at the reservation desk is that the max length for anyplace but fishing bridge is 40 ft combined length max. There are few spot you could probably fit it, but they hard to find, and in the busy season hard to come by. Of course you can't reserve them either. Also, in the last couple years the park rangers have been driving around ticketing people who have a tire off the pavement be it the RV or the tow vehicle. Years ago I never saw anyone having issues of that sort.