Rent for a week will cost you about what the Ferry would, and I don't ever remember seeing campers for rent. Cheapest option would be to park the trailer on the mainland and hotel it while on the island. Depending on what the truck costs for crossing, might be just as cheap to leave it on the mainland too and rent a car. They run about $30 a day on the Island these days.
So out of curiosity thought I would check for rental campers. Got to love Google. - Found Campers, but they all come with the truck. About $140 a night.
https://www.rvezy.com/rv-search?Types=TruckCamper
https://www.canadream.com/rv-rental/