According to the Intelipower 9100 manual- excessive load can cause low output voltage. But excessive load coming from one circuit should blow that circuits fuse. It could be that the batteries are drawing high current and the addition of more current draw through the fuse block puts it over a current limit point. The next if is, if all the lights are turned off, does it still drop the voltage, if so then you have at least a partial short.
If you leave that fuse out and check that everything else on 12V is working, then that would prove the converter is capable of supplying power. The vent fans and fridge pull enough current to test it at lower currents.
Local auto parts place may have a battery tester that can be adjusted to different loads to test it.
I've tested my PD9140A up to about 35 amps by connecting an inverter to it's output and trying different 120v appliances to draw current from the converter. Trying not to overload the inverter, I didn't find anything that would pull the full 40 amps.
As far as schematics, you may have to draw your own. I've done that for may TC and it really makes electrical issues easy to troubleshoot.