Actually it isn't the isolator, as I found out later.
The solenoid isn't receiving a switched 12V.... if I supply 12v the solenoid switches on and all terminals on the isolator show a high 13v reading.
I've got some wire chasing to do... I doubt it's something as nice and simple as a blown fuse.... none of the glass fuses in the fuse box are bad... there have been numerous pieces of wiring that have been done horribly, for instance:
3 wires for a ground... each have an eyelet... then they were bolted together with a 1" screw, dangling above the battery... enough tape to cover the eyelets but the screw was exposed - with the vibrations it actually tore through the sheathing of one of the grounds connected to it.
Another was.... all within 10" of the battery..... eyelet, way oversized home wiring nut, fuse holder, way oversized home wiring nut, butt connector with 1/8" of wire sticking out on either end (stripped back too long).
I've gone through and replaced all of the additional fuses someone else added with new wires. I've also gone through and replaced many of the shorter run wires along with many of the longer wire connections (due to corrosion).
Today I plan to pull the starter battery out and start going through some of the wiring on that end to see what's actually going on.
For the weekend (plan to head out mid afternoon tomorrow) if I haven't resolved the issue I will bypass it and use a switch with a relay (or something) to ensure both batteries charge while driving.
I do appreciate the comments, though.
