Onan/Cummins burnt up

We have (well had) a 5500 in our 2013 Itasca motor home. We were using the genny with just one air unit and the refer on when my son yelled fire. The genny was smoking bad, when I got there it had shut down and fire was out. All the wiring on the outside looks fine, I think it is the winding's.
Has anyone else had a problem?