Welcome to the forum.
I believe at the time, Winnebago published and provided these with a new motorhome, 1) a generic operator's manual, 2) a chassis manual from the chassis manufacturer, and 3) a binder that was comprised of individual manuals supplied by the manufacturers of the individual appliances and equipment installed in the motorhome.
It sounds as if you have the binder?
If that binder does not have the "generic" operator's manual, you can
go HERE to download the PDF. it's the 1990 operator's manual for multiple models but it will contain the instrument panel controls on the dash for each model so yours should be included in that collection. It will also give you general instructions on plumbing, the LP gas system, some elementary info' on the electrical system, TV, A/C, etc., etc. ...again, it's just a general (and generic) operator's manual. I'll post the full length link if the one above is corrupt:
1990 Winnebago/Itasca Opertor's Manual (PDF)
You can also find manuals and service recommendations, etc. for the chassis items online but, again, each chassis manufacturer that Winnebago used back then (primarily Ford and Chevy) will have slightly different manuals.