ST7,
Do you want to actually build a camper out of this Airstream? Or, is it going to be a single location home while you build your other house? The reason I ask is....if you're not planning to pull this trailer around from campground to campground, but rather just park it in one spot and live in it while you build your permanent house (at least until your house is done), you don't need to have ANY holding tanks.
If it's going to be stationary, just run the plumbing through some traps, route them all to a central drain, and run them into the septic system that you're going to have to have for your house. You won't need holding tanks because you're not taking it anywhere.
As well, if you don't plan on towing the trailer around, then who cares about frame sag or separation. It becomes a non-issue to a stationary trailer. Just put the jacks down, maybe stick an extra scizzors jack under the back bumper, skirt around it to keep the cold air out, and call it good.
Now, after your house is done, and you want to use this Airstream as your travel machine, then by all means do all the necessary and proper repairs. But if you plan to park it one spot for the next several years, you could avoid a lot of these headaches altogether.
It IS a lot of work to move the bathroom. But, if you're completely gutting the coach, then it's less of a big deal. You might want to just stick in an actual house tub/shower unit for now....as the RV units are about half scale replicas.
My own coach has the split bathroom in the middle and I really like it. We had the twin beds in the rear, one on either side of the aisle. I thought they were awful, so built a little bridge across he aisle and put a King mattress back there. Now I sleep like a King
Best of luck,