This is just another example of Dill's great storytelling in To Kill a Mockingbird, and it's difficult to tell when he is telling the truth or not. At the beginning of Chapter 12, Scout receives a letter from Dill explaining that he will not be coming to Maycomb that summer. He "had a new father"--whether the man had actually married Dill's mom remains unknown--and the two were planning to build a boat together. The fact that Dill claimed that his new dad was also a lawyer--"like Atticus, only much younger"--should have made Scout question the truth of his letter. Dill also told Scout that he would eventually return to marry her, and Scout was too much in puppy love for her to see that all was not right with Dill.
It wasn't long before Dill showed up in Maycomb: He had run away from home. This time the story was different: His new father, who he now hated, had chained him in the basement, where he had been forced to live on raw field peas provided by a sympathetic farmer. According to Dill, he had broken free from the chains, become a camel washer in a small animal show, and had taken a train and then hitchhiked his way to Maycomb. Dill was extremely dirty, so Jem and Scout must have assumed that at least the latter part of his story was true.
It is clear that Dill was an imaginative boy capable of telling the biggest whoppers imaginable. But he is also a lonely boy whose parents have no time for him. He is happiest when he is in Maycomb, with his two friends who accept him for his warm personality and mischievous attitude toward life.