The Matagorda Island is separate from the Matagorda Pennisula where the LCRA park is located. There is no ferry to Matagorda Island, but you can usually get a ride over from Port O'Connor. You have to pack in everything and pack everything out.
Maps of penisula from the air.
Maps of island from the air.
I would suggest you take a trip over there for the afternoon or day to check it out before you commit to camping.