We travelled around So Utah in April and sort of assumed we could make it up as we went along. We had some reservations along the way, but mostly not. BIG mistake. We ended up with some rather unpleasant RV parks (RV parks in that area are mostly gravel lots) or leaving very, very early in the morning to get to a place that is first come (of course, in some areas you have no choice - Fruita in Capitol Reef is like that and well worth the visit)
I would like to go to the area again, and this time I know where I want to stay and will be online the minute reservations open up.