Magnatek 6345 upgrade if i dont charge in storage?

Another option would be the PD9260. It will fit in the same slot as the Magnetek and use the old fuse panel. It is capable of supplying 60A but your single battery probably won't accept that much current even at 14.4v. Of course, the battery won't see the full 14.4v due to the resistance of the wire between the converter and the battery. You may be able to find the PD9260 about the same price as the 9245.

Edit: Here is a link to instructions for replacing the Magnetek with the PD9260. Clicky