Very first thought is that the wiring for the volts and the oil pressure have been mixed, the volt gage is wired to the sender for the oil pressure and the oil gage is wired as the volt gage .. You say it is wired correct as per instructions, it maybe be but it would not be the first time a set of instructions were wrong.
Color of a wire for instance may have changed and the instructions were never corrected type of thing...
Follow the wire from the oil gage it should go right to the sender. It is not a bad ground the water temp operation is telling you that.
The volt meter would need a hot lead on one side and a ground on the other, check to make sure it is not running down to the oil pressure switch. I think that is the kind of thing you are looking for..