Hi and welcome to the Forums! Congratulations on being a '70's Overlander owner; we're kind of fond of ours.
I hate to be the rain on the parade, but somebody has to answer your questions honestly, just as mine were answered when I was at your point in the process.
Unless your trailer spent its life in a barn, you have rear end leak damage. The design of the "flashing" behind the trunk lid makes it behave more like a funnel, directing water right under the plywood subfloor. The full extent of the damage is only determined by dropping the belly pan, at least in the rear.
Your black tank picture shows some changes made by a PO, which deepens the mystery of what you might find when you drop the belly pan.
The axle, unless it is a replacement, is over 20 years past its useful life and must be replaced.
Regarding the plumbing and valves, most folks don't trust the original copper at that age and replace with pex. The service manual you ordered will help here. Those valves at the rear hatch are for opening the aft drains for the water lines.
If your trailer is useable as is, you could just replace the running gear and use it a while to see what you want the end result to be and enjoy it a while, just postponing all that inevitable repair.
The good news is the forums will have the answer to every problem you face and is full of good, creative, helpful vintage owners just like you. I posted some of my repairs of this type here: http://www.airforums.com/forums/f4/s...-a-119687.html
And I borrowed a lot from Top's "Abby gets some separation..." thread. The "Full Monty" threads are invaluable, too.
Ask a lot of questions, post a lot of pictures, and above all, have fun...