I am about 90% sure I am going to be rebuilding the connection from the grey tank to the gate valve.
As far as I can tell, the eccentric reducer should slip over the collar of the grey tank drain, but it has about about a good 1/8" to 1/4" separation and appears to be connected only by whatever sealant was gooped on when they built it (or someone else repaired it.
Just have to figure out what I can use to seal it if I replace the reducer. I am sure something has to go in there to keep it from leaking. The tricky part will be if I just replace the one side of the gate valve to get the bolt holes to line up before the ABS glue locks it in place.