I can't speak for everyone, but I personally use the Good Sam Club website to lookup campgrounds and you can pick what criteria your looking for in a campground. I have found it to be very useful. Depending on the time of year and which direction your headed on your trip? If your headed towards the beach during the summer months, you had better have a reservation, especially on a holiday weekend or you more that likely won't be staying there. I typically research the area that we will venture to and find a campground that best meets our needs and then make a reservation. If you like adventure? You could always show up and hope for a spot other than a Walmart parking lot..
Happy RV'ing