Most Onan generators for RV use have low oil level cut-out switches, so check the oil level. As with any gas engine -- check fuel, spark, and compression...
If this is a large RV Onan genset that is diesel fueled, also check for any air in the fuel lines. Compression tests on a diesel requires a special gauge, since the pressures are much higher than an automotive gas engine...
2007 Tour Master T40C
Acura MDX toad
"It takes a great deal of time to recover from any improvement..."