I remember reading somewhere that one of the rejected designs for Ghostbusters II was a “portable containment unit” that resembled an oil tanker.

wasn't one of the snippets of script for GB3 that Proton Charging (was it there or somewhere else?) released for people to read - something to do with a massive facility of ECUs? Or am I remembering something that isn't real again?

In the RGB Episode “ The Copycat” it is revealed that Ray and Egon have been working on a dimensional portal to use in the future when the containment unit becomes full. The portal would allow the Ghostbusters to send ghosts back to their own world. The portal was never used outside this episode, so I am guessing the writers felt the premise of how the ECU works and as Fritz outlined in his post was good enough for us viewers.