It all depends:
Where do you prefer to stay, resorts, state parks, any place with Electricity?
When, middle of the week, weekend, holiday?
Geography, Florida beach, Yellowstone, any place in Nebraska?
As Don pointed out, RVing is growing rapidly. I have no data to back this up, but my guess would be they are building more RVs than they are building camp sites, especially when state parks are struggling for budget. What was true a couple of years ago may not hold true today.
My thinking is that if you know where you are going to be on a particular date, why not make a reservation. If you are just traveling through, and any place with electricity is OK, you can take a chance. Like others said, stopping early gives you more options.