When booking a reservation I print out my receipt. Armed with that the park has no chance of charging me dime one more than my receipt shows (just another of the many reasons I NEVER stay at KOA's with all of their unmentioned additional fees.... but I digress). If they persist in trying to charge you additional fees, nicely/politely, appeal up the chain of command but be prepared to walk (drive), away. Assure them them that you will report the charge to the fraud department of your credit card company, do so and then get on the net and tell the world about their thieving ways.
What they are doing is fraud which is a crime. Be prepared to convince them that you WILL make a federal case about it (even if you aren't). Paying any additional charges simply makes you an accomplice to their crime by enabling their fraud and encouraging them to continue trying to do it to others.
As always.... opinions and YMMV.
:C
