An RV Onan generator over-heating - due purely to any natural ambient air temperatures that one would find in the U.S. - doesn't quite "sound right" to me.
These RV generators cost around $3K on up, so they should be able to take natural high ambient air temperatues if the oil used in them is the grade that Onan recommends for high temperatures. I'll bet that built-in Onan vehicle generators are used around the world, including in the hottest places on Earth.
I wonder if some marginal part in your generator didn't cause it to fail just from long hours of running, per se?
A couple of other thoughts: If you ran it while going down the road ... where is the Onan's radiator located relative to the scorching air washing off behind the RV's hot engine while underway? Due to this, perhaps the air coming in around your Onan's radiator was in fact way higher than the ambient air temperatures. Adding to this, the ambient air temperatures for several feet above hot highway concrete/blacktop surfaces are going to naturally be much higher than the ambient air temperatures above the ground at campsites.
Then again, perhaps if our summers are going to average hotter and hotter, RV generator designers will need to up their game such that their generators - if properly maintained - can be used 24/7 for long periods regardless of ambient air temperatures in a campsite or when going down the road.
2005 E450 Itasca 24V Class C