To remove and replace a battery make sure that you are not plugged into shore power. Then make sure you have everything in the trailer shut off. The storage switch will most likely do this for you as well. When you go to remove the cables from the battery always remember this simple phase:
By this I mean to remove the negative cable from the battery first, then remove the positive cable. This will prevent you from shorting out your wrench to the frame of the trailer.
When you install a battery, install the positive cable first, then the negative for the same reason.