There are various battery switches which go on the battery terminals, used mostly by vintage car collectors. The problem with the Airstream battery box is that it has very little extra room inside it to accommodate such a switch. A second issue is that often the switches are not very high quality. And, hate to say it, but if you have the inverter option, you most likely have two sets of heavy cables going to your batteries, one for the normal coach loads and the other for the inverter. Both must be disconnected (usually on the negative side of the batteries).
I like the Perko brand marine battery switches, but they will not fit inside the battery box and even outside mounting is tricky and requires extending cable lengths.
On my FC 20' I put a pair of golf cart batteries in to replace the original #24's which required a lip to deepen the battery box. What I do to disconnect the batteries totally for storage is to remove the connecting cable between the batteries, as it is the easiest one to get to and only takes a couple of minutes and two nuts to be removed.
On my old Argosy, which has batteries inside the trailer, I use a Perko switch as there is a lot more room to work with.
Wish I had a good easy answer for you, but I have not found one.