Most of the 'stuff' in your RV was made by other manufacturers, not National, which you know is out of business. Write down the make and model of each system in your RV and do a Google search for 'MAKE MODEL # manual'
and you'll probably be able to find manuals, troubleshooting guides and even parts lists.
Most owner's manuals are so generic and full of "how to wash your seat belts" and other pearls of wisdom they don't give much real info.
You can look here and see if they have one for your RV:
Manuals - 1990 & Newer - RVing-The Midwest Connection
But if you download some make and model manuals you'll see a pattern. Winnebago also has a pretty complete set of documents you can download, their manuals can give general tips and behaviors.
You should use a multimeter to determine if both chassis and house batteries charge when on engine alternator, generator, and shore power. Batteries fully charged should read about 12.6 v at rest and if a charging voltage is present, the meter should read at 13.5 v or better. This information is good to store away if in the future things don't work as they should.