Tripac APU (legacy model) not charging batteries

First make sure the ground wire isn't broke off alternator on apu very common problem. then make sure the ground wire going to the back of engine is good grab on it and shake. Both those wires break because of the vibrations. the sure the controller is set to monitor your battery it can be turn off and you leaving it stand by mode? Get the manuals on line learn your machine. Has for your heater there all kind of video's on you tube showing you how to pull that heater and clean it and replace the screen just do it and save a ton of money