Marty,
There are several possibilities here. First, there could be a bad temperature sensor that was taken off your negative battery post (intended location), and just stuck back in a corner. Second, and I have seen this, the inverter was replaced and the shorted control cable for the old inverter was plugged into the battery temp sensor port.
As you can see, the inverter will function well without at battery temp sensor, but if you want to spend a little time (depending on distance and obstructions between the inverter and batteries) and a few dollars on installing a new temp sensor, the inverter can charge your batteries at the optimum rate for their temperature which can be a benefit to you batteries and extend their life, especially in a high temperature environment. However, if you spend all of your time on full hookups in a location that never gets above 70 degrees F, you do not need a battery temp sensor.
Gerald