Our ONAN 4KY runs the fuel pump to prime the carb when it's not running and I hold the Start-Run-Stop switch in Stop. I don't think yours does that but I don't excel at reading schematics. The narrative didn't indicate it did.
What you want to do should work. I'd suggest though, that you disconnect the fuel pump wire from the harness then apply the 12VDC to just the pump. Then you know you aren't backfeeding something you shouldn't. A jumper lead protected with a fuse or breaker is a good idea. I use one with a 10A auto-reset breaker.
You can get a more definitive answer by asking Moderator to move it to Tech Issues, then update with the full Model/Spec number. Last character of Model is Spec.