We are retired with an empty nest. As such, we do nearly all of our travels during the Spring and Fall when the folks with school age kids are stuck at home, which makes it much easier to get campsites. We never make advance reservations for long trips of 6-9 weeks, as we have no idea where we will be after the first few nights. For popular destinations such as Yellowstone, we call ahead a few days to a week ahead to reserve a site. There have been a few rare occasions when we had to make a second phone call.
For us, making reservations for a long trip would spoil the freedom of RVing. We often stop to see some unexpected discovery along the way, decide in the morning to skip a site that we had been thinking of visiting, decide to stay an extra day or leave a day early.
For us, reservations would be a cause of stress, not a relief from stress.