I have a 1986 Itasca motorhome in which I have installed a 2001 Chevrolet 8100 Vortec Mag HO Engine from a Mercury inboard set up.
My problem is with the wiring loom and the fact that the plugs appear to be wrong. As the loom is from a truck I thought that a marine electrical loom may be more suitable. The loom would be stand alone – engine only – but with a fly by wire throttle pedal.
Sounds like a fun project.
The inboard engine should have all of the electronics required to run mounted directly to it. The only wires needed to get it to run would be 12 volt power from a key switch (usually purple on the engine wiring) and a momentary start wire from the key switch (usually yellow/red). Coming from the engine, the gray is the tachometer lead, the light blue is the oil pressure, and the tan is the water temperature.
I don’t have experience with fly by wire throttle. Most of the Merc engines in that era were a straight cable to the accelerator. Good luck with that part.
If you can find it, ebasicpower has a ton of short engine harnesses and boat wiring harnesses that may make your adaptor magic easier to accomplish.