Is this legal???

We often take future reservations prior to our setting the rates. The price is not guaranteed until those future rates are set. That is very clearly noted in the confirmation. I would suspect there is a similar disclaimer somewhere in the OP's reservation confirmation. Also, as noted by another poster, taxes can change (and I have never seen them go down) and the park would be in the right to pass those increases along since the tax obligation is generated at each day's stay. (can't pay them in advance).

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..

I've had parks advise me of a price change for future bookings, but existing bookings have always been honored at the originally quoted price. In your situation, I think I'd weigh the value of my planned stay against the price increase, and decide whether cancelling had a higher or lower value to me. Be aware that a cancellation fee could cost as much as the increase though.

I would ask for a refund if they insisted on a change of price.
Best to read the fine print of the reservation.

I would do as toedtoes suggested. If that fails, you either pay the extra $80, or ask for a full refund.

Because you are so far away, going to court in NH for breach of contract is totally impractical. No, it is not right. They would be in breach of contract (unless they had something in writing at the time you paid which said they had the right to raise rates without notice on prepaid reservations), but it isn't worthwhile for you to pursue it.

Personally, I guess I would ask for the refund first if I had anywhere else I could stay instead, simply because I would not want to give my business to such underhanded people.

Can they do it? Sure.

Is it good business sense? Probably not.

What I would do is call them directly and be very very very nice while asking them to honor the original rate.