The SH-500-50 is a current shunt that is used by the the TM-2030 battery monitor by Bogart Engineering to measure your RV’s electricity usage. A shunt is the component that measures how much current flows into or out of the battery bank. The SH-500-50 shunt is recommended for most medium size RVs with electrical loads not exceeding 400 A. In most typical RVs the greatest loads develop when a microwave is used. Typical RV microwaves are rated at 900 – 1,200 W of cooking power and 1,000 – 1,500 W of power consumption, so they will pull approximately 100-120 A from 12 volt batteries. Therefore, a 500 ampere shunt is the appropriate size. If you never use a microwave or any other high power device, a 100 or a 200 ampere shunt may be sufficient. A smaller shunt can provide finer current resolution, which can be fun to watch and overanalyze.
However, for practical and wide range use, a 500 A shunt is probably the most versatile.