I think it all depends upon the time of the year you are traveling. Between Memorial day weekend in the Spring and Labor Day weekend at the end of Summer is very "iffy" getting into many campgrounds without reservations. I guess it really all depends on how popular particular parks are. And others have already stated alternatives to RV campgrounds, which we have never used.
Ourselves, we never make reservations accept for Custer SP in SD in September. Booked solid every year.
Over the years, we have found when traveling west of the Mississippi River, the more options for an overnight stay there was. A lot more small RV camps along side of small businesses.
The beauty part about traveling without reservations is that if we like a place or find interesting things around the area, we stay awhile, if not we continue on. :)