The biggest mistake you can make RVing is having to be in a certain place at a certain time.
Disagree. For those of us with limited time it's sometimes the only alternative. While it does make for some stress at times, arriving at a destination and not being able to find a place to stay can be more stressful. "Structure" for some when traveling is not a dirty word.
I'm a planner so I always made reservations. I don't like surprises, especially bad ones! I also didn't have the luxury of traveling much other than peak season. If you are traveling off-season or in a not-so-sought-after area then reservations probably aren't necessary. During peak seasons or in popular places reservations are probably recommendated.