Hi from Colorado and welcome to Air Forums and the vintage Airstream hobby. There is a lot to learn, and these Forums sure helped me learn.
Maybe your 66 Caravel
is like my 66 Trade Wind. Your owner's manual probably talks about the gopher hole waste dump method. That isn't done any more!
Your trailer, if like mine, has a rather inadequate black water tank under the toilet. In fact, the toilet is rather inadequate also. Mine was leaking and thus rotted out the plywood floor under the toilet. And the sewer hose connection process involved getting on your hands and knees, opening the hatch, fishing the hose through, connecting it, and then opening the dump valve. Very unhandy in my view.
And you may find that your black water dump valve is likely shot too. At least mine was. They are 50 year old trailers for goodness sakes.
Here is a photo of the black water tank and dump valve. The tank is so thin it is very likely to clog up. I ended up replacing mine and rebuilding the entire bathroom.