Get a gas can with some cleaner in it. Hook it up to the pump. See if you can get it to run by holding the choke closed a little. Or if it will run let it run on the cleaner mixture.
Your carb is clogged up. The Onan dealer is going want you to buy a new carb. At over two hundred dollars. If you are good with engines. You can remove the carb and clean it. Just be careful with the gaskets.
Onan tech since 1980