You might search here: National RV Archives
Even if they don't have your exact model or year, most manuals aren't that specific or change that often. Even if you got a Winnebago manual (They have a great web resource) the general instructions can be most useful. I'd also suggest you make a list of make and model of each appliance and system in your RV. If you Google for the manufacturer, you can often download manuals, parts lists, etc. for the items in your RV. It's actually better than the National SeaBreeze Manual, they tend to be rather general when it comes to specific operations of appliances and systems in your RV.
Winnebago manuals here:
Manuals & Diagrams
This website has good ideas and many manuals:
Manuals - 1990 & Newer - RVing-The Midwest Connection