Good question. I would have enjoyed having a service manual for my '67 Overlander, but, like you, found one is not published.
On the other hand, I have completed some fairly significant tasks on my Airstream without a service manual. And, I doubt the information that would have really been useful would have been included. For example, my bathroom decking had rotted to wet sawdust. A dimensioned sketch of the curves, and plumbing access holes would have helped the restoration effort. But I doubt that would be covered in the service manual.
Stick with this forum for help; All in all, the collective experience you will find here will get you through your tasks.