I was thinking that a bad contact on L2 might cause a voltage drop and a corresponding increase in amps that could trip the breaker. I suspect L2 because L1 has to have reasonably good voltage to actuate the contactor.
Try turning off the main and individual breakers in the coach circuit breaker panel and see if the generator breaker will reset. If it works with no load, turn off the working breaker on the generator so you can test voltage coming out of the suspect circuit. In the coach breaker box, turn on mains and a single outlet breaker and measure an outlet between hot-neutral, hot-ground, and neutral-ground. If you're not getting very close to 120/120/0, that indicates a problem. You might have to try several breakers to find one that is using L2.