I like ReserveAmerica, it's a centralized place to make reservations when traveling from one place to another. At one time, we had to call each State Park ahead of time to reserve a spot and each SP had different policies.
Some SP would not even take reservations and it was all "first come first serve" which sucked when you were 200 miles away and arrive late in the day.
Now I log onto RA - see what is available for that night and book online. I no longer have any anxiety worrying if we will find a place.
The cancellation fee is a small price to pay for the convenience.