He testifies to his innocence. He never saw the girl that day. She was never in his motor home to his knowledge. He went on a trip and got stuck. He drove around and came home. He did not kill her. What's the prosecution going to use as a counter? His fingerprints on her body? His DNA on her? Nothing.
If he does not sway the jury, they will convict him. It's just that close.
The defendant never has to testify in his own behalf. But most attorneys will tell you that it ever-so-little counts against them if they do not get up on the stand.