Here are a couple examples of GMC motorhome owners manuals. Like a lot of things in the vehicle, the manuals changed multiple times over the 6 years that the GMCs were made.
1974 GMC Owners Manual (X-7321C)
1977 GMC Owners Manual (X-7721A)
An example of the ever changing vehicle can be seen in this Coachman owners manual for the Royale and Birchaven models. Since the manual below is from 1977, it does not include the center kitchen floorplan that was only offered in 1978.
1977 Coachmen Owners Manual