I would imagine that the alternator wire goes to a battery isolator if it also charges the house batteries. If it does you may want to start checking there. You can disconnect from the isolator and reconnect direct to the batteries then recheck the voltage. Make sure you check for a good ground. It is important for you to know your charging system and know how the alt. is excited and then the path of the output. Many people have thought the Alt. was bad because no output not always so. There are many good articles on the web if you find your config. then troubleshoot and get it right the 1st time.