My dad has a '77 Jayco that he's owned for going on 20 years. He's never had the original manual. Obviously you are handy and not afraid to tackle this project. What I'm saying is that you really don't need the manual.
At one point we opened up a corner of his trailer to repair a leak, which involved removing the front couch/bed and replacing the rotted wood in the the walls and floor.
Construction on these is all essentially the same. Just take your time taking things apart and note where everything connects. You'll be able to find equivalents to everything at your local home supply store.
Wiring in these is really simple. Everything is either 12 volt in series or 110. RVs are not subject to building code so the wiring is not complicated. As you take apart the roof just look where the wiring goes and follow it below the cabinets.
Have fun, that's quite a project!
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE