there are 4 house batteries different compartment, next to the inverter and converter/charger. One 12V Starter deep cycle battery and one huge chassis battery.
When you mention a huge chassis battery that may be the starting battery. Many diesel chassis of that era used a 8D starting battery which is about sixteen inches long X 8inches wide X about eight inches high. These dimensions are just a rough guess from memory as it has been many years since I used them on diesel generator sets.
I understand the four house batteries but do not see the reason for the small 12 volt deep cycle battery. I do not recall every seeing a deep cycle battery used to start a diesel engine.
Check the voltage on that large battery as that may be related to your problem.