Are you sure it's ground?
Disconnect the battery cables. Put your multimeter on OHMS. Measure from each cable to frame ground, and ONLY attach cables to the outside negative battery post that have continuity to ground.
If you have levelers, you should (could) have two cables on the outside battery positive and one cable on the negative. The negative cable should go directly to frame ground.
Are you sure the batteries are installed in correct polarity?.
Can you measure 6V across each battery with everything disconnected?
