Hi and welcome to Air Forums. Maybe you have two vintage Airstreams, a 71 and now this 68. I have a 66 Trade Wind which is similar to your 68. The 68 is better as you have better window design, and a significantly better bath layout.
I have the owners manual for my 66. It appears Airstream had the owners manual printed in advance and applied it to all the Airstream models sold. My manual is pretty general and doesn't have a lot of specific information in it.
It does have a wiring diagram and a plumbing diagram. I have used those on occasion. Travel trailers are pretty common in their systems. My owners manual does not talk about specifics of the furnace or water heater for example.
You will find a lot of information in the Airstream Knowledge Base toward the bottom of the Forums page. Click on Tradewind, and then click on the year range. You can read about folks who have renovated or restored these old trailers and can really see how they were built and what is a good approach to renovating them. It takes a lot of reading, but that's how you will learn all about your 68 Trade Wind. It worked for me. And remember, the 66 through 68 trailer were pretty similar. A 22 foot Safari is very similar to a 24 foot Trade Wind is very similar to a 26 foot Overlander. The floor layouts were different, but the construction was very similar.
Hope this helps...