This is news to me. I have never heard of a gen running so poorly on 87 that you had to run 91 or it dies. All the small engines I have ever had, generator, lawn mowers, motorcycles, tillers etc, have always run on any old gas I had around and have never shown preference for any octane (though the Moto cross motorcycle does have more power in high octane)..
I would look elsewhere for over-heating problems before looking at gas.
What gen do you have?
Did u check for a mouse nest or other debris on the cooling fins? The Onans are enclosed so mice or rodents can build nest in there and cause a hot run.
Oil at the right level?
Correct oil, usually SAE 30?
So, Im curious... someone mentioned it so, what happens when a gen runs on that E85 gas? I would never even consider it, but if I had no choice this is an option?
I only ever use 100% gas. Solar is king for me unless I am running AC.