RV campground and a airline ticket are two entirely different things.
Apples and oranges comparison.
Airlines are heavily regulated operations, RV parks not so much.
Airplanes falling out of the sky, a big deal, price hike to park your RV on a small plot of land, not so much.
Typically a RV park in a extremely popular destination are often hit up by local governing bodies (city) for taxes. One place we go (a popular beach destination) typically gets hit with increased "bed taxes" on a yearly basis.. Campground owners are not going to absorb the tax increases so it is ALWAYS passed on to the consumer.
Likewise, campground operators face increasing costs to operate like power, water, sewer, labor and maintenance.. Those costs will then be passed along to you the user.
Nothing illegal for the campground requesting additional funds after you have reserved and most likely if you read their fine print they might even spell out that additional charges may apply..