swapping and charging batteries

I have a 1 year old trailer. It came with a deep cell battery, and it is good. I plan on adding a second battery. I have 2 older ones,that at the time, were good. Today,I swapped out my good battery for one of the older ones to let it change for a week or so with the trailer's charging system. When I plugged it in,it was at 12.5V. I have not charged it since sometime last summer, or the fall before. Not bad...