San Luis Pass County park. Just on the south side of the toll bridge. They have a RV park there. $10 to dump.

There's an RV park on Bluewater Highway going toward the town of Surfside. Try talking to the owner there. He may be hesitant due to some past issues, but he may let you dump there.

There is a campground a couple miles away, look on google maps. its a little north of the beach itself.