I'm a bit confused mate. What and where are you measuring? Chassis should be earthed (-ve) and to measure you would check between +ve and chassis. If you are seeing +ve on the roo bar etc, what earth are you using?
I think the oil pressure and water temp gauges are both 12v and they have a converter circuit behind the dash somewhere. Probably only a centre tapped ballast resistor. If this had shorted it might allow 24v to chassis - not sure. I'm trying to be helpful but don't know much about the 24v Patrol. My experience is on a 24v Mazda/Ford truck a few years ago.
14.7 volts on your second battery is pretty high if it is not switched on and under charge. That battery could be high resistance internally. If you can get a load tester try each battery seperatly and check the discharge.
Some trucks use a resistor to split the two batteries to get 12v as mentioned above. That can cause one battery to fail more often than the other and also can upset the charge system.