Most KOA's charge $10 if more than 24 hours notice. All cancellation rules are clearly posted on the reservation page for that particular KOA franchise.
I had to cancel and forfeit a $34 fee this summer due to health issues. The campground allowed me to reschedule, which I did even though I thought I probably wouldn't make it to Jacob Lake this year. Sure enough after 11 weeks on the road my wife didn't want to go back out again soon after we got home. I called the campground to cancel just so they could re-rent the space. I want this campground to stay in business since it is the only FHU park even half way close to the north rim.