What could be the issue?

Like button ^

1. The Genset itself is NOT the problem
2. The Genset has NO charging capability. THAT is done thru the Power Converter or Inverter/Charger which ever you have.
3. I would suspect the Battery Positive cable between the fuse and the connection at the genset is melted/crimped/shorted. OR routed close to the Genset exhaust OR Engine Exhaust(I pick the Engine exhaust). That is why the fuse is blowing after running and the Geset and Engine exhaust gets HOT. Good thing the OEM installed the that fuse or you would have a good fire on a shorted 12 volt Positive cable. Doug

Probably near the house battery compartment.

That doesn't make sense! Blowing a 100-amp fuse type load could burn out your whole electrical system.
It must be an electrical battery wiring problem. I would locate and careful inspect the Aux start relay circuit. It normally is same circuit as battery charge circuit.

That doesn't make sense! Blowing a 100-amp fuse type load could burn out your whole electrical system.
It must be an electrical battery wiring problem. I would locate and careful inspect the Aux start relay circuit.

@ Jim - That’s a good possibility. It does charge the batteries when driving. So it may be a back feed/over charging issue that is causing the fuse issue. I have no idea to confirm this. Where would a switch be?

@ envlethen - It is fine when the truck isn’t running. But when the truck engine is on, that’s when it blows the fuse.

Is the 100-amp fuse in the cable feeding the genset?
Is it connected to the positive post on the battery?
What do you mean by it is fine when genset is going, then blows when it is on?
If genset is running, there should very little 12-volts DC on this cable.
Only possibility is vibration from genset is causing cable to be grounded out.

I'd try unplugging the battery charger if there is a plug for it, or add a disconnect so that AC charger won't charge the battery.

If there are separate batteries for the house/vehicle, you should add a switch or switch it so that the house battery is independent/not charging the vehicle battery. The switch is what a lot of RVs have.

I believe what's happening is that both the car and generator are charging your batteries and you are getting feedback, but that's just a guess.

Fwiw, I blew out a circuit on a Onan generator by starting it while plugged into shore power. So, be careful with your trouble shooting.

This is a class c motorhome. The batteries for the home part (rv portion) is under the step. Under the step there’s two wire that are for the generator. There’s a 100amp buss amg fuse. It’s totally fine when the engine is off and the generator is going. But when turned on, the fuse is tripped from anywhere from 10 minutes to three hours (three hours being the longest, usually it’s within 5-10 minutes)