We camped in southwest Colorado last year. We were surprised at how many RV's were on the road and in the parks. We made reservations in advance but it was at an RV Park. Normally we enjoy camping at National Forest Service campgrounds but those all close by September 30. They do have first-come-first-serve sites, however.
This year we are camping in Gunnison and we made our reservations in January at another RV park.
I think it depends on when you go. Before we retired we would try to find a first come site on a Sunday afternoon. That worked really well for us.
This year many families bought or rented RV's for family vacations. Then again, many people like us I suppose, will travel in the fall. So not sure what the situations will be like.