Unless you upgrade your converter/charger to 3 stage, I would buy the cheapest deep cycle group 24 (I think that's the size) I could find. Even then, know that the charger/converter in your Airstream will cook the battery.
Computers manufactured by companies such as IBM, Compaq and millions of others are by far the most popular with about 70 million machines in use worldwide. Macintosh fans note that cockroaches are far more numerous than humans and that numbers alone do not denote a higher life form. -NY Times 11/91