I had a similar problem finding the chassis wiring for mine. You are correct that Winnebago can supply everything you need for the coach side, but it is difficult on the chassis side.
It may be difficult to find a manual for a unit that old, but give eBay a try. I have a 1998 chassis and have the set of GM chassis manuals on the way now. You might also try an established truck service facility in your area. They may have the manuals for your vehicle and let you at least look at them. In some cases you might find a truck service center that will sell you copies of the pages you need. Worth a try.
A bit lower on the page you will find a Winnebago owners forum. Try posting there also, there are still a good number of vintage Winnebagos on the road today.
Helm publishing is the publisher for many of the auto industry manuals. They might still have them available, but they are pricey from them.
BTW - welcome to the forum. Keep posting and asking the questions, someone here probably has the correct answer for you.