The height of your vehicle would be a problem in the nearby multi storey's,  and even if you can find anywhere the costs of long term parking would be hefty around that area as its all geared towards short stay shoppers/day trip visitors.

If the hotel can't help you the cheapest option is probably to speak to someone like skypark on the airport estate and leave it over there, they might even drop you off and pick you up at the ferry terminal as part of the deal, if not a shared taxi wouldn't be too much, the airport isn't that far from the ferry terminal by taxi.