You can try searching for a carburetor repair shop but I think I'm dating myself and they're probably all out of business these days. The varnish from that old gas has plugged the jet passageways (evidently what was known as the low speed jets - on a generator like the Onan, they would be light load jets as the generator must run at a constant speed to provide 60Hz power). Buy some carburetor cleaner, disassemble the carburetor and let is soak for several hours in the solution. Lacquer thinner may be able to soften those deposits but the carburetor is formulated to do the job. After removing and rinsing the parts, blow out every passageway with as high a pressure as possible before reassembling it. Then give it another try. Make sure the hose between the fuel pump and the gas tank isn't cracked from old age and allowing air to be pumped into the carburetor instead of gas from the tank - this is a common problem on RVs over 10 years old.
Good luck with your efforts.
