Reservations don't work very well for us because we typically don't know where we're going or when we'll be there. It depends on the season though. I wouldn't freewheel it during busy tourist seasons, but in the fall when most people are back to work & school it worked very well for us.
The night before a travel day, my wife and I would agree on a general compass point and vague destination. Then I would quietly research potential stops along that route. RV Trip Wizard has a feature that shows rings of distances, so I would look at stops around 300 miles away.
The next day on the road, I would prod my wife to start looking for places to stay. If she didn't have any thoughts, I'd suggest XYZ or ABC parks near some town. She'd either agree that they sounded good or pick someplace else, and then she'd call and book it when we were a couple hours away.
We only panicked one night when there was a midweek auto show in the small town in the Sierras where we were stopping. We still got a good spot in an offbeat park. That's the only time we've sweated getting a spot in our tens of thousands of miles of traveling.