It might take a lot of luck to get a manual for that specific year and there are times when the manuals are too generic to get down to real helpful detains, so I might suggest a different line of chasing info. Winnebago does settle on what works and keep it for many years, until there is something major that changes. So it may be just as easy to go with the closest year posted online which is 1981 and try to sort what items fit what you have.
Many thing like how the water pump works will be the same but it is true that there are lots of small changes as time goes on.
But if one starts with the oldest manual you find online and looks at gradually newer for that model, you may find the right info on a much newer manual?
Not going to be easy in many cases but you must have known that already!
One would not want to use a manual for a current phone to sort out trouble on a phone from 1977!
And then it is true that there are likely to be folks on here who have owned several of the older RV and might be able to point the right direction on some things. I think I had a 1976 brave at one point???
For the water pump, try looking in the compartments and next to the water tank? If it is working, turn it on and listen for the noise?
Generally the older RV are much less prone to having things that are automatic and harder to fix. Not a lot of the "convenience" type stuff to break down.
No outside satallite TV, no power door locks, no inverter to watch TV while out in the boonies? How in the world did we ever survive when the neighbors or us didn't have a generator to keep us awake all night?
Maybe that's why there are more mass shootings now?