The shunt is used for monitoring the current going into (charging) or coming out (consumption) of the batteries. It's really a high-current, low resistance link where an electronic monitor can read the voltage differences on either side to measure the current flowing through it.
I think the Trimetric units are pretty popular in RVs. I have one from a company called Microlog that provides similar features, but bang for the buck is probably not as nice as the Trimetric.
03 Chevrolet Silverado 1500, 5.3 V8, 3.73
06 Rockwood Roo 23SS