Hi, '65 Caravel
owner here. I just replaced the rotted plywood underneath the rear wall of the sink/shower/toilet area and am beginning to button everything up. Needless to say I had to remove the fiberglass insert and toilet and rest of the plumbing. My problem is the replacement size of the water line.
The grey water from sinks and shower stays in the drain pipes until you remove the cap from the line as you stated. The black water from the toilet stays in the holding tank (small) until you open the thetford valve which allows it to flow out. The grey water line runs to the thetford valve and connects to it below the black water valve area.
In my Caravel
the toilet opening of the black tank on top and thetford valve opening on the bottom of the black water tank are offset by a few inches. You can close the thetford valve and the black water will not flow out but you could take the cap off the waste line and have the grey water run out. (boondocking). Hope this was helpful.