lol, I'm sitting up here in Connecticut thinking to myself "Wow, $40? that's cheap!"...
Assuming the campground didn't cut the camper off from using the bath facilities, laundry, pool, and whatever else - then that is a a good price for a "family resort" campground around my neck of the woods. I assume also he had access to a dump station with fresh water to refill his water tank.
Local campground near me, if you just need to empty your tanks, charges $20 JUST TO DUMP if you don't have a campsite. Spending an extra $20 to spend the night and have access to bathroom, showers, and laundry doesn't seem to outrageous for a one time only deal.
Yes, $40 seems a lot to spend the night in a parking lot (which is where I assume the overflow spot was), but sometimes it happens. If they charged you $36, would it be better? and would you come back here and talk about the wonderful KOA that helped you out for one night? Or is $34.50 better?
It might have been better if you had come on here to start a post simply letting people know how much that particular KOA charges for overflow, as opposed to ranting about them.
And... assuming you were coming off a long day of driving, you were tired and cranky, and just needed a place to overnight for a few hours, I can see getting a bit uppity at the price.