I just went out this morning and took another look at it, measurements, and pictures. It looks like it's a 2 inch pipe coming out of the bottom of the tanks.
View of Both Tanks and how they are connected:

Closeup view of the connection coming from one of the tanks, it's leaking from where the upper part of the white pipes meets the tank. So does it look possible to replace, or what's the best way to seal it? I'm not certain, but it looks like the bottom of the tank might be threaded, and that I might be able to cut off the T and unscrew the lower threaded connection from both tanks, remove upper from tank and re-seal, then put/glue back together.. Does anybody know whether the tank itself is threaded?

And just as a reference, this is the Black and Grey tanks more towards the front of the trailer:

Dave