Cancellation fees are a fact of life at many RV parks. If you do not want to pay the fee and are not absolutely sure about the timing, then best not to make reservations. And hope you can find something else in the area.
Changing arrival dates can also be problematic in popular areas. Calling to change the arrival date to three days earlier or later than originally planned and you may find they are already booked for those days and you are out of luck for that park.
As noted, you can almost always find some place in the general area to stay, but it may not be the most desirable place to stay. And in worse case, you may be in a Walmart parking lot.
