We kinda do this every season on this forum. No one is in the parks, then first week of January parks are almost full. Then middle of March they are shrinking at a very fast rate.
Because the topic comes up every season I think those that have been doing it for awhile have a "gut" feeling there is a change in the RV snowbird lifestyle. I've had that "gut" feeling for awhile now. Parks get full later, and empty out sooner.
IMO, each new wave of retiree's want more and more creature comforts. Living in a tin can on a lot that is 35 feet wide and mostly concrete or gravel is not cutting it for many. Unless you grew up on a farm (which is a shrinking lifestyle also)the average person is not comfortable driving and backing up a big rig. Hard for most to jump from a 4 door sedan which they drove to work for 40 years to a 40ft DP pulling a toad or a truck/5'ver combo. Guessing that most US people retire at 65 because of medical costs. Not exactly the time in life to start learning how to be an OTR truck driver.
Myself, we stayed north late this year because of a new baby in the family. I am surprised how comfortable I am right now without that burning desire to head south. Then again we've had a wonderful fall.