I have found Motor manuals be be superior even though they cover many models instead of one huge poor manual devoted to a single one (Haynes). Admittidly, they don't have wiring diagrams or instructions on how to service the AC vents in the dash, but those would not apply to a motorhome anyway. The engine and running gear sections are what you need, and Motor does a great job.
Check out this one on Amazon from 1982 (Amazon.com: Motor Truck Repair Manual (9780878515509): Motor: Books
). It would cover yours. The main thing is to get one that is old enough. Even though they still make 454s, you don't need info on the new fuel injection units and that vintage ignition systems.