4000 onan

I had the same problem with an Onan 4000 and it turned out to be the ignition coil. It was very frustrating to figure out because the code, I think it was code 36, said something like an uncommanded shut down.
The unit has to be removed from the motorhome and tore down a little to replace the coil. I finally did a SWAG on it, replaced the coil and it ran fine again.