I just got our 4000 back from the Cummins/Onan shop south of Atlanta. I had same error code 36 and my fuel pump was bad.
Also, to the guys saying to use synthetic, the Onan rep at the shop told me NOT to use synthetic. He said the particles were smaller and it had a greater chance of losing oil. He said the best oil was regular 15w40 or the straight up 30 weight.
If you look on Onan website they have a ton of service centers. You can call them and order anything you might need
2015 XLR Nitro 30FQSL
2015 F250 King Ranch