I totally get electrical not being your thing. You might be better off to either get it in to your RV dealer or call Onan direct, there are two in Utah SLC 801-355-6500 and Vernal - I'm not sure those Onan shops will serve you as they don't show RV as a market served, so your dealer might be your best first call.
If you want to keep going, you need to trace the 12v line from the generator back to the battery and/or switch. With an ohmmeter you can tell if you are getting 12v power to the generator.
You can also try disconnecting the 12v lines from the generator and using another 12v battery to jump the generator. If it starts and runs you've isolated the issue outside of the generator.