Had what sounds like same issue you are having last summer.Turned out my transfer switch was the culprit. I was at Ky speedway so using generator was a must.The contacts in mine were burnt pretty bad.
I removed mine completely and hard wired a 30 amp plug in generator compartment and now when I use generator I run power cord into the compartment and start generator.
I have an older coach so I don't mind making changes like this may not want to do yours this way .AT this time I have no plans of replacing the Transfer switch