I just replaced the OEM batteries in our 2006. I got way more life out of them than I expected. They were conventional lead-acid so I will assume Airstream is still using them in the 2012's.
I have been a long convinced user of Optima batteries in our cars and boats. The are excellent. Never a failure. They work in all conditions and can sit uncharged. I even replaced the dual batteries in our old diesel with a single Optima Redtop.
However the price has gone sky high (almost $200) so I put in 2 Duracell AGM batteries from Sam's Club that were only $80 apiece. The Duracell and Optima are both AGM. The optima batteries put the glass mats in a "jelly roll" which holds up much better to vibration (like in a boat). But there is not much abuse in the Airstream. So we will see.
Take careful measurements of your battery box and you may be able to go to a larger battery. The reserve capacity is what you are interested in, not cranking amps.
AGM batteries like the Optima have no liquid acid and can be used inside the car/trailer. Car audio enthusiasts routinely mount them in the trunk and cabin. They also can be mounted sideways or even upside down so if you want to add more to your Airstream, look around under a bunk or closet if the outside battery box does not lend itself to expansion.
A car Audio shop (there must be thousands around San Jose
) would probably be willing to add an AGM battery or two if you are not comfortable with the task yourself. Pick one installing in $50,000+ trucks and cars, not beatup Hondas