I would use the term "typical" rather than average, cause they vary all over the map, even within the same campground. You can probably get by with 20 ft or less 90+% of the time, but when the time comes that isn't enough, you had best be prepared. That's why the optimal configuration is a moderate length cord plus extensions on hand for use as needed.
Also, the typical RV will come with a 25-30 ft power cord, but I've seen some at 20 ft and some at 35. If you got a 50 footer with yours, that is unusual!
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL