There are very few places East of the Mississippi River where you can still just drive in without a reservation during peak Summer months. During the cold months, you can still camp at any number of state parks just by driving in. But the water in the campgorund may be turned off too.
I do not remember the last time (if ever) we just drove in. We have called ahead a few hours to KOA or GoodSam campgrounds when traveling sometimes to see if they had space available for an overnight. But the main destination always has reservations months in advance.
