Disclaimer: I'm not a lawyer, not by any stretch of the imagination, nor anything resembling an expert on consumer law as it relates to campground reservations.
What does your reservation contract state? Does it include any sort of an explicit price guarantee, or language suggesting the price may change, or neither? Assuming it doesn't specifically guarantee the price and/or the specific site, I think legally they're very well within their rights to change such specifics with notice. I'm not suggesting it's a nice thing to do, nor necessarily a good business practice.
Assuming they're giving you the option to cancel the reservation with a refund of any pre-payment, and also that there's no wording in the contract that guarantees the fees and site, then I don't think they're behaving unreasonably--just perhaps inadvisably.