What is the problem with measuring the 1200A with the shunt?
50mV to measure is easy - an low noise opamp (you must use +5V and -5V power for the opamp) with amplification set to ~50 will do that. You will get 0-2.5V for 0-1200A, for example. Filter the input voltage a little bit and that is all.
You need a DC-DC converter to get -5V and a single opamp, 2-3 resistors, 2-3 capacitors.
You may isolate the whole stuff by a separate battery (so the only wires will go to the voltage terminals of the shunt - use shielded cables) and send the data to your truck control center via Bluetooth 