Two more things.
First, I did find a recall on that model year, so you probably should ask Winnebago if that work was done.
https://www.recallowl.com/Vehicle+Re.../D20+MWRG/1975
Second, the oldest Winnebago manual I could easily find on line was this one for several 1981 models. Not all that useful (but neither are the manuals for my 2019).
https://winnebagoind.com/resources/m...ago-Itasca.pdf
Finally, you might be wise to just search for the engine/drivetrain manuals from Ford/GM or Chrysler, depending on who the manufacturer was.