I would be willing to bet that the issue is the fuel pump on the generator. I have the same issue on my 5500. Onan put such a small fuel pump on them that it is insufficient at bringing the required amount of fuel forward from a rear tank. It is only a 2psi(approx) pump and low volume at that. Here is my solution: 1. Go to Oreilly's and pick up a Mr. Gasket 1-3psi fuel pump to replace the factory Onan pump. 2. Also pick up the other Mr. Gasket pump that is 4-6 psi to add to the fuel line as close to the fuel tank as possible. 3. Tie into the wires from the factory Onan pump to also feed the rear newly added pump(make sure wire size is appropriate for the length of run). Now when the gen calls for fuel it will turn on both pumps and the rear pump will push fuel up to the little 2psi pump on the gen. You may be able to get away with not replacing the factory pump but just add the rear pump. There are a ton of threads out there about this issue and a factory replacement pump is $110.00. The Mr. Gasket pump is half that and looks very similiar. This is just my opinion and my fix.
2012 Colorado 4x4 5.3v8 CCSB
2005 Sierra F37SP...soon to be gone
2005 Suzuki Hayabusa....wish someone would buy me...
1971 Camaro restomod in progress...yeah right....