I am not certain from your question precisely what diagrams you wish to obtain. Diagrams for the various systems of your Overlander can be found in the 1976 Factory Service Manual . . . a copy can be purchased from the Airstream Factory Store at this link
. If you are seeking floorplan layouts, these are available as pdf documents at this link
While the current listing of service manuals on Airstream's site doesn't list 1976 versions, I am certain that a call to the store would reveal whether one is available along with whether a 1975 or 1977 would be a closer match if the 1976 service manual isn't available. Typically, if you just need one or two diagrams and you post your precise needs here on the Forum, you will find a member who is willing to share a pdf copy of their diagram.
Good luck with your investigation!