If you call Monaco and give them your VIN, they may be able to find something. However, for a coach this old, it is unlikely they would have much.
One big advantage may be that your coach wasn't wired with wiring harnesses, but rather by running individual circuits, something like a house is wired. (That's the way my 1993 HR Navigator is wired.) That would apply to the house, not the engine or chassis, but Cummins (or Cat) might have some wiring diagrams. Call the engine maker with the engine serial number.
Spartan has been helpful in the past with specifics on components used on my chassis. I don't recall what identifying information they wanted to identify my chassis.
Holiday Rambler Navigator DP, Hummer, and Honda VT1100C Shadow