It's almost impossible for a 13xx to get full like that.
I would suspect that at some stage the Nuvi was connected to a computer that had a nasty virus that has placed additional files into the device.
Usually these files are hidden and system. If you can find these hidden files sometimes using the command. attrib -h -s -r -a will reset the file so that you can delete it.
Make a full backup including the hidden .System folder before deleting any files.