OK, it may be out of date for you, but we just took a trip with quite warm temperatures and ran out generator for at least 4 hours. No problems so I feel comfortable that the loose wire to the transfer switch solved our problem. Hope you have already fixed yours by now, but if not, check out the wiring for loose connections.
We just returned from a Thanksgiving trip and quite honestly did not attempt to run the genset and A/Cs enough to make a final determination. We're still keeping out fingers crossed that the fix was the loose wire because on the way back from Lone Star I ran the genset and A/Cs for maybe 45 minutes and in the past had experienced problems by then. Lone Star claims to have one of the best Onan guys in the country and they are an authorized Onan Cummins repair facility according to Lone Star, so if your problem is in the generator itself, they could also be the ticket to a fix for your issues.