We are ready to embark on our full-timing adventures starting in July. I have followed this forum as well as many other blogs over the years and find that many people do not make advanced reservation...
You will learn from experience when to make reservations. Depends on the area, time of year and type of campground. For example, Florida southern and costal state parks; even now some are full for parts of next winter. Really big national park campgrounds without hook-ups usually don't fill early. I guess, when in doubt, reserve?