Who routed you to them?
If it was the campground website, it most certainly is the campgrounds problem as you used their system. If their system was wrong, it's their problem.
If it was some other 3rd party source routing you to them, they still are pretty much on the hook from a PR perspective in that they accept reservations from the company. They might be off the hook from a legal perspective.
You can always got he credit card route but the problem is once you accepted the site, that may be taken as tacit agreement to change the terms of the contract.
Tammy & Mike
Ford F250 V10
2021 Gray Wolf
Gemini Catamaran 34'
Full Time spliting time between boat and RV