From owning three 2800 Microlites which are similar, I think there's a knockout-type access plug located in the case below the service door. Remove it and the wire boot and you should be able to get a deep plug socket into the plug well. Replace the case plug when done, since it's part of the seal for the cooling air.
(on edit: if the camper structure interferes with access to the plug well, you may have to loosen the mounting bolts and tilt the front up & back.)
But if it's stopping immediately when you release the switch, I doubt it's the spark plug. The small Onans get their ignition voltage from the battery when cranking, but switch over to self-generated voltage once the switch is released. It's likely you have a failed voltage regulator or other issue in the voltage-making process (brushes or rotor/stator).
There's a pretty good step-by-step troubleshooting guide in the factory manual, assuming you have a volt-ohm meter and can use it. Flight Systems has some good info online too, IIRC.
Jim, "Caution! Do not look into laser beam with remaining eye!"
