The print on the Xantrex Prowatt Sw inverter (as well as the one for the Morningstar SureSine) call for connecting the + and - to the battery and the chassis lug to ground. The Morningstar being more descriptive says use earth ground for the chassis and mentions making the ground wire larger then the battery cables. This supports Jons hookup but doesn't address using the frame as a conductor for the -.