and Welcome to the Forum!
Don't worry too much about an "Owner's Manual" for the coach. They usually contain very generic information. Wiring or plumbing schematics are typically non-existent, especially on something that old.
Start finding the identification tags for all the appliances and systems, Furnace, fridge, water heater, etc. You should be able to find those service manuals online. This web site should get you started.
Service Documents and Manuals
Depending on what chassis you have under that coach you can probably find an Owner's or Service chassis manual online too.
Best of Luck!