You need to be careful with an Onan on a unit of that age. They didn't switch from carburetors to EFI until about 2005. If the unit hasn't been cycled under load regularly during the off-season, you could have significant problems.
My '02 rig has an Onan 5.5 Kva generator, which is probably an '01 build at Onan. It only had 49 hours on the Hobbs meter when we bought it and it started instantaneously and ran just fine. In our second winter season, I needed cardiac surgery, and my downtime meant that I didn't get out the the rig to do the regular exercising for about 12 weeks. That was in early spring 2012, and the generator hasn't run since.
I suspect the carburetor is toast. I'll replace it, and if still doesn't run, it's time for professional help. It's a nuisance, because we're FHU campers. The only time we needed the generator was to run the second a/c on a very hot day in eastern Washington. Unfortunately trying to sell the rig with a non-functional generator will be a struggle.