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What is the actual model number of your coach? That will allow me to pull up the wiring diagrams for you which will be useful for any kind of electrical issues you might have.
To your actual issue here.............I don't believe there is. The only thing between the battery and genset should be the disconnect solenoid. That does have some fuses on it. One way to tell if that is bad is that it controls ALL the 12 VDC circuits inside as well. IF those work, then this is something else. It may be in the genset. There is a fuse on the control board. Checking this will take either a 12 VDC test light or small digital multimeter like you can find at most hardware stores for about $20 or so. Get back to me with your model number and I can direct you further here.
If the batteries were dry, they are going to be ruined. No amount of charging will change that. What you heard may have been the converter going out. When there isn't any battery and only converter output for 12 VDC power, it can overload it when a heavy draw like a genset motor and blow the fuses in there or even ruin it. Start with the batteries and have them load tested and eliminate that part of the problem. Then we can test the converter.
Just so you know.............fully charged 12 VDC batteries should read 12.8 VDC. When the voltage is low, the amperage spikes and that creates heat in the circuits and components and blows fuses and breakers and damages controls and things.