The idea that you can just jump in your RV and head off on vacation is overstated. Most of the popular State Parks are booked - often a year in advance. People go to the same parks, year after year, and even stay on the same sites. At 12:01 on New Year's day, they log onto the Park website and reserve their site for the next year.
And no, I'm not kidding. Popular State Parks fill up fast, and the idea that you can just drive off to a park and they will have a site is a bit of fantasy. And yet, in most locations, rarely do all the hotel and motel rooms sell out.
For example, getting into the popular First Landing State Park in Virginia Beach, Virginia, can be problematic, particularly on weekends. People literally make reservations a year in advance, and on weekends, often every site is sold out. Sometimes you can wait by the gate past reservation time and see if there are "no shows". If there aren't, well, you're sleeping at the Flying-J or Walmart tonight!
If you want to stay at the beautiful Hunting Island State Park in South Carolina - or the confusingly similarly named Huntington Beach State Park, reservations are also in order - particularly on the weekends. Again, sometimes you can wait by the gate and find a cancellation - but you can't count on that.
For longer trips, the idea of going to campgrounds and finding them all full would, of course, be frustrating. So many folks make reservations for every leg of their trip. We did this on one trip out West, and it was, to say the least, not spontaneous. If we saw a place we wanted to stay, well, we couldn't stay there - because we had another 200 miles to drive to our reserved campsite!
There are, of course, less popular and less populated campgrounds that may not need reservations - and in some cases, there is a reason for this, which you discover once you arrive. But it is possible to be spontaneous, particularly if you are not traveling on the weekends.
And if you are planning a trip, call the park and ask if you need a reservation. In many cases the park ranger will tell you that it may not be necessary - that for the time you are traveling, the park is rarely full.
RVing can be a lot of fun, but there is a lot of misinformation and mythology associated with it. And much of this mythology is spread by the RV industry, which has a vested interest in selling you on the idea that spending $100,000 on a motorhome is a sound proposition.
Be realistic and know what to expect. That's all I'm saying!