Eddie,
Knowing the actual model of the Onan would help, but there are two things that could do as you describe (expecting that the facts as presented are accurate).
First is that most of the Onans use the flyeheel alternator to provide the run signal that says its cookin'. If this is open (not uncommon) or just a bad connection the control board doesn't know it is running or at least should be....
Second is a control board issue. Without knowing the model so I can look it up all I can tell you is that replacements are dear. Onan is so proud of those boards.
Matt