I limit our daily hours behind the wheel to 4 or less and so we have time to make stops along the way. We have a short RV so we have less to worry about with campgrounds that are first come first serve. If we had a rig that was longer than 28 feet there would be fewer campsites available and a much greater need to reserve one in advance into which the RV would fit.
For example there is campground at a state park east of Phoenix where fewer than 10% of the campsites are pull through ones where a long trailer or a motorhome with a towed vehicle can easily fit.
I only make reservations where there is not another campground in the area. If there are 3 campgrounds then we have options is one is full. So it all depends.