Adding 12 Volt receptacle in back storage area

The power center usually has some spare spaces available to add circuits. If you use one of the spare spaces you can add a dedicated circuit and fuse size as needed, this would be a shorter run then going to the battery. You wouldn't be trying to add a fuse block and trying to make the connection at the battery. The power center is usually near the floor so that you could run under the trailer to the back.