Advantage of reservations, you know you have a place for the night.
Disadvantage of reservations, you have to be at a certain place come night or lose your reservation fee even if there are delays along the way.
Advantage of no reservation, flexible in how far you drive and what time you decide to stop.
Disadvantage of no reservation, may have a problem finding a site or at least a place you would feel comfortable staying.
As a minimum, while driving and when I have a feeling of generally where I expect to stop for the night i look up RV parks in the area and call ahead to reserve a site. The earlier the better as others will also be looking for places toward the end of the day.
Many use parking lots, truck stops, and rest areas for flexibility. But not for me.