Had the same issue with a Cummins diesel. Turn the key, nothing. Lights didn't dim either. Over the years paid a handful of money for techs to work on the problem and even after replacing the starting motor, it persisted (though it started quickly with the new motor, allowing the mechanic to call it good). Since I was always traveling, could never get a handle on it nor could the techs that worked on it. Even paid for a starting motor rebuild down in Mexico.
So, over 12 years with the problem, and a few hundred dollars spent, never found what it was. Seemed to be heat related and if I just WAITED for 1/2 hour, it would crank right over. Gave up and lived with it until I sold the RV.