I just went through a similar ordeal on ours. a 97 Pace Arrow with an Onan 5000 Marquis. The genset only has ~620 hours on it, so barely used. The genset is a "post emissions" unit with a sealed throttle. There is NO adjustment for the throttle, it is all automatic.
It had sat unused for years with the previous owners who always used the shore power. It started out not running at all. We took it apart, cleaned it, replaced the filter, seafoamed it etc and got it to at least start. Once it did start, it would run very rich and smokey and eventually die after a few minutes. Took it apart several more times, worked on the choke, etc. Could not get it to run properly at all.
I ended up just buying a new carb as the newer (post 96/97 or so) carbs are "not serviceable". I think I paid about $300 for it and ended up being a better solution in the long run for my gen set. The reasoning behind it is that the new emissions carbs have too many tiny air and fuel passages to ever get fully clean once they get gummed up.
Good luck with it!