Powering Arduino from same battery it is sensing (Positively grounded)
I think with a few modifications you could do this rather easily. One method is to get an isolated DC to DC converter. This one will do exactly what you want without any other modifications. However, there is another option - hang the microcontroller power on the 12 volt rail, and then use a 7905 or similar negative voltage regulator to supply the microcontroller ground. You'll have to rework your voltage dividers, but this would also work in a pinch. I would recommend just using an isolated converter though.