First you have to check for spark to the plugs.
If not check the source......points dirty???? condensation in distributor??
Second to check if its getting gas through the carburetor.
Carburetor may need cleaning if its not getting gas.
Agree with DL. You must run a generator monthly to keep it clean inside the carb, etc. The unit I have is an old 89 31 ft Winnebago that had a generator in it but only had 6 hours on it (the couple never used it) and I could not get it started. had to have the carb rebuilt, etc. I run it now each month like DL said and it starts just fine and runs smoothly. :laugh:
I know that you are supposed to run your generator (under load) monthly, but I admit to not doing that with my Honda EU3000. Maybe its a better design than most, maybe due to me shutting it down by turning off the gas and letting it die, or maybe I'm just flat out lucky, but it always starts right up on the rare occasions I use it.
On the new motorhome, you better believe I'll find some way to exercise that expensive Onan monthly.
I used to to that same thing with my Onan before we went fulltime. It has an electric fuel pump and it connected with a spade connector. When we would return from a trip, I just pulled that plug on the fuel pump and ran it until it stopped. But with the modern Onan that is harder to do. The other reason that Onan says to run them under laod is to dry out any moisture that may collect in the windings of the generator. That would be true of you generator as well, but it is something that usually doesn't happen for a long time, unless you are where the humidity is high.