It's not that you will have a problem pulling in somewhere to overnight, it's the quality of the place. If you just want to sleep anywhere, sure, you could wing it unless you are in certain parts of the US. (Some parts of the NE have minimal public land campgrounds and not many commercial ones either. Out west, we've driven long desert stretches recently where there were no open rest stops and it was many, many miles to a gas station. Wouldn't have wanted to be out there as darkness came, hoping for a place.) So at least plan ahead during the same day.
But if you like state and national parks in high demand places, enjoy great views out your windows, waterfront sites, excellent and clean commercial campground facilities? You might consider planning ahead.
Back during the recession, it was easier to book close in than it is now. We notice dozens of shiny, new RVs daily on the roads and in the CGs...lots of new retirees come of age daily. More competition for sites.
For me, the atmosphere at the campground or RV Park is part of my enjoyment of a vacation, and I enjoy the research. I like knowing where we'll sleep and spend a few days in special places. But I'm a planner and others consider it confining. So it's an individual choice.