Always post the ONAN Model Number with Spec Code. Spec is generally the last character of Model Number.
If you have a Model beginning "4KY" then it's a single cylinder 3600-RPM generator. Removing the genny from the coach won't help much with the sparkplug unless it's in a really odd mounting location.
AND... If it IS actually a 4KY, then holding the control switch against STOP (before you try to start it) should run the fuel pump. Try it from the outside switch, the one on the genny. You have to remove that access cover. The end of the switch that doesn't have the red indicator is STOP. Hold it in STOP and you should hear the fuel pump. It's a tapping sound, not a whirr like an electric motor. If you don't hear it, something's wrong. It should run, tapping pretty loud (easy to hear from beside the genny, maybe not from inside the coach). Then it should taper off and go quiet. If it doesn't get quiet, it's either bad or not getting gas from the coach's tank.
Try this and let us know what you find.
