I'll have to defer to the vintage Airstream guys as to what will fit, but if you intend to "boondock" i.e. camp without electrical hookups, I buy TWO of the largest batteries that will fit. Group 31
12 volt batteries are only 1/4" longer than the Group 27 that fit most compartments. Two in parallel give 260AH.
If the compartment is tall enough, two T-105 golf cart batteries will be a little cheaper, and give more discharge cycles (i.e. longer life) at the same discharge depth.