When you change the alternator controller from the original AlphaPro to a AlphaPro II or AlphaPro III you will find an enclosed temp sensor in the package. You can configure the alternator controller to have the temp sensor monitor the alternator instead of the batteries.
In my case I also added a Mastershunt to the system. This shunt also comes with a sensor and broadcasts the battery temperature across the Masterbus network. This way you can feed both the battery and alternator temperature to the AlphaPro. It also enables some more control over the AphaPro.
I'm not at the point of installing Lithium given the charge/discharge/SOC complexity but I did prepare my electric system as far as I could to install Lithium at some point in the future. Mastervolt is very much double in it's advice to monitor the alternator temperature. I've seen publications where they do advise to put a sensor on the alternator. In my case it was simple as it did not require additional things to buy. Knowing the alternator temperature is also useful to anticipate on some of the problems that may develop (worn out bearings etc.)