In times past, it use to be after Labor Day, State Parks and even private campgrounds became ghost towns. Public schools use to start their new year the day after labor day and end around the middle of May. Kids were out of school the day before Christmas and returned January 2. Fall break consisted of 1 day off for an item called "Teacher's Institute." Same in the Spring. No week long spring breaks and such.
We are fortunate to get campsites every week-end somewhere in Indiana, even last week-end, but we are utterly amazed that State Parks in Indiana are filled almost to capacity! Folks are camping longer, and later in the year now. What gives?
I think it's partly due to more RV's yes. But also the changing and shifting school year for our kids. This year schools started back up the first week in August through Indiana. My gosh!
With that said, those with campers, and those with young families are learning to adapt. If they purchase an RV, the get out now, regardless of the school year, until weather is prohibitive any longer to make it feasible (snow states).
But, to respond to the OP's thought, yes, it's happening everywhere.
Give it time though. Folks are finding out that paying the price for RV storage, a loan on a camper that is sitting in storage, and the inability to go anywhere because everyone else has their RV out of storage and beat you to the campground, that eventually more and more people will be giving up their campers, and the the tide will turn again. Campgrounds will become empty again and reservations and even walk-in will be welcome.
Meanwhile, I feel your pain. Sometimes we have to really search to find an open campsite in Indiana. But, somehow we always do. It might not be the number 1 prime campsite, but we can usually get in.
Case in point... Site 40 at Summit Lake State Park in Indiana has to be the best campsite in the entire park. We've tried to get a reservation on that spot for 3 years. We got lucky several months ago and was able to make a reservation for site 40 almost 6 months ahead. It was booked every week-end the entire Summer (3 years in a row). There were other available sites, but it took us 3 years to get site 40. We were there 2 weekends ago.
Winter camping is just around the corner. State parks in Indiana turn the water off around November 1. Campgrounds really clear out then. Looking forward to doing some "dry" camping in November now!