Batteries are catalogued into group numbers. We have had 2 Airstreams, both a little more recent then yours and both took a group 27 battery. This is a standard size for many automotive uses and is readily available. The only limitation is the size of the compartment you have. Double check especially if modifications to your trailer have allowed larger / smaller batteries. Our compartments are original and will not allow a larger battery. In general for our uses larger is better but within the limits allowed. When in doubt take the old one with you or measure it's dimensions to be sure. Also note the connection. I have found both a traditional battery post connection as well as a threaded post with wing nut. Some batteries are made to handle both.
Lots of information here on the forums regarding batteries and our choices available. Check around but be prepared to be confused before your done reading.
Roger in NJ
" Democracy is the worst form of government. Except for all the rest"
Winston Churchill 1948
TAC - NJ 18