Sounds like a fuel problem, possibly the mixture leaning out as the choke opens.. Start it up and let it run until it dies. Close the choke about half way with your fingers and try to start it again. If it starts, you've found the problem, if it doesn't, take off the air cleaner, spray a little carb cleaner into the carb with the choke open. If it starts, you have a fuel problem, if not, you have to dig deeper.
The fuel problem could be anything from bad gas to a bad fuel pump. However, if you have to go deeper, try this web site..http://www.smokstak.com/forum/forumdisplay.php?f=6
They might have the repair manuals and someone might be able to guide you through troubleshooting. If you need parts, Onan is now part of the "Cummins" family. Be sure they don't ship "Red Lable", it's hard on the wallet. If you call Cummins, you might be lucky enough to get a parts list from them.
Have a good one,