Hi Dave,

Rather than a short, it seems you may have a broken connection from
the accessory battery negative to the chassis.  Without this
connection, any small load or leakage current (such as through the
12V-5V converter in your newly added USB port) would cause the
positive of the battery to float near ground, and, especially if you
are using a high-impedance digital meter, this will result in exactly
the symptoms you described.

If you want to play hot-plug-cowboy, ground the battery negative to
chassis and see if that fixes things.  Be ready to disconnect if the
spark is too big or things get hot.  For a little more caution, you
could try making the connection through a fuse (say 10-30A).  If this
does resolve the problem, then follow the original connection from
battery negative to chassis and look for the break.

Let us know how it goes,

Alex