Jim: I've experienced the same thing, at least in the few places that I have stayed. They have what they call off-season rates or discounts that lead into early June and then again in mid-September or so. So, during the high summer season, the rates are generally higher. For the same cabin, I've seen it as low as $50.00 to about a hundred +. Can't say that I've ever seen a $300.00 jump for the busy season - that must be a very popular place and they can get that premium from their customers. Other places have the same rate, regardless of the time of year.
Docking fees seem to be gaining popularity and are usually around 10 to 15 per day ... or 50 to 75 per week. For a weekly rental, I've seen a bit of a discount. I don't mind the charge if the docks are kept up and in good/safe order. Most places I've stayed have very well-kept docks. Others ... well, not all the docking fee is put back into the dock.
Also, for the launching and parking, when I am staying on-site, I've never been charged for these. This assumes they have their own boat launch and parking facility. I have stayed places where the camp does have it's own ... so we have to pay for the service.
Surcharges for pets are also increasing as the number of camps that permit pets seems to be dwindling. We have two dogs that go with us on all trips, so my list of camps is not that great. Some places charge for them, and others do not.
To this point, I've not encountered any surprise charges, once I reach the camp. I've been very lucky with all charges, in writing, and up front. I receive a reservation confirmation, with all the charges and taxes clearly marked. To this point, I've not hand any problems with the camps that I have stayed at.
Take your time in selecting the camp and don't hesitate to contact the camp directly and ask specific questions and see of you can get things in writing. It's your trip and your holiday ... and you'll want to enjoy it to the full. Lots of great and knowledgable folks on this board that will be more than happy to help you out.