I've not specifically worked on a 79 motorhome but based on the ones that I have worked on when Airstream assembled these things they started in the back and then worked their way forward.
That doesn't necessarily mean you have to rip everything out in front of the shower but it does mean you will have to remove something, possibly a closet wall, to get to it. It's kinda of like a jigsaw puzzle, if you enjoy those then working on your Airstream will be fun. Otherwise.....
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)