Dread Island by Joe R. Lansdale
One of the stories in "Classics Multiated", which is a collection of stories that mash together literary classics with supernatural elements. I read the special signed edition of Lansdale's story from Subterranean Press. The story uses Twain's characters (Huck and Jim mainly, though Tom Sawyer and Becky make appearances) and throws in Lovecraft and Uncle Remus, with a island that appears in the middle of the Mississippi river at certain times.
In many ways it's typical Lansdale, but he does capture Twain's voice fairly well I think.