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!"
'06 Tiger CX 'C Minus' on a Silverado 2500HD 4x4, 8.1 & Allison (aka 'Loafer's Glory')