Sorry about your battery problems. Did you check the voltage before you installed them in the Airstream? I would probably charge them up before putting them in storage and then check the voltage before taking them out of storage. If the voltage has dropped any, then I would charge them up before installing in the Airstream and wait a day or two to make sure they are holding a charge. If they do not hold a charge, then they need to be replaced.
I suspect part of the problem is that you don't know what these batteries went through before you got the trailer. Now that you have new batteries, assuming your converter is working properly, you should be in pretty good shape.