Onan oil safety

I know little about the rig I just bought and a lot that sell donated vehicles. The sales man told me he had it running for a day or two, but then it shut down and would not start again. I have ran it for about 30 seconds while holding down the start button. But stops as soon as you released. There is over a half tank of fuel in the tank. New fuel. I would have to say it may have been setting for years. It hasn't be registered sense 2006. The sales man informed me he was able to rum the AC on the rig for several hours.

Thanks for the help

Gordon