We haven't visited these islands but our previous RV visits to other islands were not a problem with a 40' motorhome and car. I see by their website that they take commercial vehicles so that probably means large trucks. The largest ferries, such as those to Newfoundland and Alaska, can take logging trucks while middle size ones can take just about any RV. Your best bet is to call them and ask. We've found people at ferries to be very helpful. By the way, they charge by the foot and your rig will be measured at the dock because they have to calculate weights, lengths, and widths very carefully to have a balanced load. You'll have a great time watching the loading and unloading!
ArdraF