Most often the reason a breaker trips is because there is a huge overload in the circuit. A circuit consists of the power from the battery, through the switch, to the generator starter, through the starter, and back through the frame of the generator, through the frame of the motorhome, and back to the negative on the battery.
Somewhere along that path, there is likely a weak connection. It acts like a load to the circuit, and just causes a lot of heat to the connection, the wiring and the lugs that hold it all together.
Just find it, that's all that is most likely wrong.