Randy,
From your question, I can not tell you much. There are numerous possibilities that could be causing your problem. The simplest and cheapest possibility would be that the unit has not run in a good while and additional priming is needed as described by David earlier. There is also the possibility that your coach fuel tank is below 1/4 tank and the generator is getting no fuel. A restricted fuel filter will also give you the same condition.
However, if none of the above condition apply, you probably have a glow plug or control module problem, and they can both be expensive to repair. Unless you are a fairly good mechanic, either of these repairs need to go to the Onan service center for diagnosis and repair.
Gerald