will start but will not keep running.

could also be that your pickup line from the gas tank is bad and sucking air.

How good are you with a multimeter. The Onan troubleshooting guides are available online for free. Get your model and serial from the sticker then go here or here.

Assuming this is the Onan we are talking about:
Check the oil level first.
If it is a shared fuel tank, fill up the tank.

The most common failure is the fuel pump. When mine failed the run time got shorter and shorter until it wouldn't start at all.

P.S. In my experiences with them, the Onan/Cummins diesel service shops are competent and reasonably priced.

That is the Cummins diesel truck service centers, NOT any RV service dealer.

does the rv have over quarter tank fuel? some gen sets won,t run if main fuel tank is less than a quarter full.

Drain the gas, change the filters, clean the plug, and try again.

How is the oil level?
Your genset could have low oil pressure switch and low oil level. Both would shut down geset.

So.. Rv or generator?