This is strange. I replied last night and the post disappeared. Anyway...
Welcome to the forums. Great to have another vintage Airstream from Mass.
We rebuilt a '65 Caravel
and spent thousands of miles traveling across the country in it. They are a great little trailer. Here
is our restoration thread. In case you haven't noticed, there is a sub-forum devoted to your year and model right here
. It's a goldmine of information. If you plan to do a lot of work and have a lot of questions, it is the best place to put a new thread. This forum is getting so large that some posts seem to get lost -- your fellow Caravel owners will be the best resource you have -- posting in that subforum will keep you in touch with that community.
Your black tank is below the toilet, in the raised portion of the one-piece fiberglass bath. It is shaped to fit the curve, has an opening under the toilet and one below that should be attached to a black tank valve for dumping. I highly recommend replacing the entire tank and valving!
You are lucky that Vintage Trailer Supply now carries all the parts you'll need. See the Plumbing Page
. These tanks weren't available when I did the work on ours. The first one I'd look at is this one
. I'd recommend measuring it very well and have them spin on the correct parts. This worked very well for me on our latest rebuild for our fresh water tank.
Unfortunately, you will have to do a lot of deconstruction to get the bath out. Inspect the floor very well. If you have rot below the walls, where the wall is bolted to the frame, it must be addressed -- this is structural. If you don't repair the damage, towing will slowly destroy your trailer.