I had a similar problem with my previous coaches Generac generator. What I found to be the problem with my generator was the brushes. I my situation one of the brushes was held in the open position (Up) by the bracket that held the 2 brushes. The brushes cost me about $15.00 plus $80.00 labor to fix..
Before finding this problem I was told so many things, I am too embarrassed to tell you my story and how much it cost me before I met a service tech who was servicing one of the Gensets at the construction site I am presently securing. He figured out the problem within a half hour.
I hope this helps, if there isn't any power coming from the generator check the simple areas such as the breakers at the genertor first.