Don't give up too easily. The campgrounds seem to think they can set the rules/laws regarding services. If you were not told up front that the fees were non-refundable, the fees can't magically "become" non-refundable. Even if they say their website has the information or it's in their brochure, if you don't have that information, they can't assume you knew. As a tip to the campground owners, you should TELL the campers when you accept the money that it's not refundable. In the case of a month long rental, they simply have a duty to inform you that it's not refundable. It can be done with a sign in the office that says "no refunds" but you can't claim it's not refundable without making sure the consumer knows it's not refundable. You can't assume everyone has been looking at your website or read your brochure either. Be fair to campers and they will be fair to you.
As a consumer you have the option of disputing the charge and dealing through the charge card company. You shouldn't have to pay for unused services. You may have to fight for it, but if you end up in a courtroom, the first question that will be asked is whether you knew and were TOLD in advance that the charge was non-refundable. If the answer is "no", you will most likely have the court on your side.