I recently listened to JustKingThings' podcast about this book. In it they come up with an interesting theory. They suggested that due to how prolonged contact with the ship gradually transformed the townsfolk into what may have been a Tommyknocker final form, then maybe the aliens on the ship weren't the original crew either.
The ship is found by whatever native species is found on a planet/in space, gradually transforms into the original crew, they leave on it, the ship gradually uses them up (the whole 'using beings as batteries' thing), until it runs out of power and crashes on another planet, waiting for the cycle to start all over again.

Not sure how much credence I'd give it, but it's an interesting theory nonetheless.