Water in battery boxes

[t_dogg80]

Hey everyone. So, we have a week until we take our new trailer on her maiden voyage. I was checking things and decided.to open up the batteries to have a look at the levels. I have 2 6v batteries. We've had it a month and it's been plugged in. Fluid levels were fine. But I noticed that the battery boxes were about 1/2 full of water all around the battery. In order to dump them out o have to disconnect the batteries.

Any tips so that this doesn't happen again? I still can't figure out how rain got in there.