Unfortunately the manufacturers don't usually provide you with much info on the systems that they "add" to the coach which they have built. Typically, they provide the manuals from each manufacturer's system. i.e. - generator, hot water heater, furnace, refrigerator, toilet etc. How this is all wired/plumbed in any given model coach is not always readily available. There are however, some places on the internet that make a business out of supplying documentation pertinent to the systems that are no longer in production. As far as the availability of information relative to the drive train and supporting components of your unit, try finding a "Chassis" or "Service" manual for it. I have a 32' Class-A 1979 Holiday Ranbler - Imperial 5000 on the Dodge M-600 chassis. It utilizes the 440-3 engine and the A-727 Loadflite transmission. I acquired my chassis manual from Irv Bishko Auto Literature (www.autobooksbishko.com) but there are others out there as well. I connecte3d with Bishko simply because his web-site used to indicate that he specialized in 70's vintage Dodge MH manuals. They aren't especially cheap, (I paid about $60.00) but when you need things like wiring diagrams they're worth every penny. You might also try www.moparmanuals.com. I believe the chassis manual that I have might be the same one you need. The manual (Part Number 81-370-8113) says it covers 1978 - 1979 Dodge Motorhomes on M-300 thru M-600 chassis. You can also connect with the Mopar Manual website via phone (203) 237-4795 if you have any questions about their manuals.
For water heaters, furnaces, converters, refrigerators and the like, do a google search includung the manufacturers name and phrases such as "service manuals". Don't forget to come back to this forum as there are numerous people out here with lots of good information. Some of it you won't find in any manual. It's called practical experience.