1992 Marathon lighting questions

I have a 1992 Marathon (electric) that I'm hooking up headlights on. I know you can't use the chassis for a ground. But that means you have to find headlights that are not grounded thru their own brackets? The headlights I have do have separate ground wires but are grounded thru the headlight brackets. I've got a 12 converter on the way. Will I be able to run the positive and negative wires to the headlights without any feedback from the chassis ground?
And why do I pick up voltage thru the chassis ground (volts depending on which battery?
Thanks for any help or suggestions!