The older Onans had a tremendous airflow that exited on the bottom. I told a fellow who was doing the same thing as you he would need lots and lots of cooling air. Three hundred degrees was too much for a small compartment. Running down the road. Dangerous.
I ordered four 39 watt axial fans to a greatly enlarged venting and strengthened compartment door. It still wasn't enough. Had to pile on the insulation on the top and inside of the compartment.
In the end he wasn't a happy camper. Good thing I had no part of it. He ended up selling the rig. This is by no means a prediction of how things will work out for you.
Personally I wouldn't advise holding your breath.
