If you get no voltage either side of the 85amp breaker/fuse seems to me that one side of it is not seeing battery voltage of 12.x and the other side is not seeing converter voltage of 13.x. So no ground anywhere?
Maybe jumper the 85a thing and see what happens to the battery post voltage
EDIT. you do get 12.3 at the fuses so that must be battery voltage, not converter voltage so it is confusing you get no voltage in places as reported. Are you getting battery voltage at the DC panel? Are the batteries at 12.3 instead of 12.7 as new? If so they should be put on a charger until you can get this converter supply sorted out.
Now that you can hear the solenoid clicking, does that light still stay on? That might mean the solenoid is moving but not making contact.
Do you get battery charging via the alternator / isolator path with the engine running?
Did you put all the wires back on when installing the new batteries? :)
No idea what it is exactly, but some threads like this mentioned a BIRD some MHs have. If you have one, is it ok?