If you can't do this test yourself, find a mechanic who will locate the line from the gas tank to the generator. Disconnect line at the generator, then have him cut the line close to the gas tank. Use about 40 lbs. of air and blow through the line towards the generator, if stuff comes out then the line is clear. If it wasn't clogged in that section of line, then blow the line into the tank. Then use a union and hose clamps to join the two lines together. They should be clear, so hook up the line to the generator and give it a try.
I have an external extended run fuel tank for my Honda 2200. I see no reason you can't do one with an Onan.