I have limited info or ideas on the BIM switch but there are some things that I can discuss about the current wiring as one of my favs to talk about!
Some things to help? One is that Winnebago stamps ID on their smaller wires.
You can then go to this chart to "decode' that stamp and find the "from" and "to" of each wire. That often leads to a hint of what to expect from each wire for it's use!
In the OEM mode solenoid, the LR wire brings 12volts from the dash area down to the coil in the solenoid. this comes from two spots at front. One is when we push a momentary switch with various names like "boost or Aux". That is the switch used to "jump start" a weak chassis battery as the solenoid connects coach and chassis batteries together!
The second time LR gets 12 volts is when the RV engine is running, a point often called "ignition hot" also powers LR to operate the solenoid as we drive.
That lets us put some charge in the coach batteries as we drive.
When LR get 12volts, it goes to a small lug on the solenoid, through a coil inside and then to ground to make the solenoid operate!
There are two versions of this solenoid, 3 or 4 posts! When you have the 3 post type LR goes on the single small post, through the coil inside and then the body of the solenoid gets ground from the mounting screws! Those screws can be flaky , so many RV also have the wire FM which is ground.
On that design, the coil inside may be brought out as a second small lug and the FM is put there to provide ground.
If you have a 3 post, Wire FM is often just laid down on the mounting screw to make a more reliable ground !
So the solenoid wiring is small wire LR on one small lug. If you have a second small lug, wire FM goes there.
If a 3 post version, FM can be run to the mounting as a ground!
Then on left and right, there are two big lugs. Left is usually the big battery cable from the chassis side and right is the battery cable from the coach batteries!
ARGH!! I just flipped over to look at the drawing you linked and I've got to say it is about the worst I've seen for this section! Hard to read, so I understand any confusion you have!
Note the triangle with 6 in it? that points tot he notes at eh left side and it tells to lay FM on the screw!
The info above is correct as far as I know but it is not clear on that drawing, for sure!!!
I think you will be wanting to leave the coach battery disconnect the same and not involve it but only swap the solenoid.
The BIM does much the same thing but is a solid state item rather than the older electronic relay of the solenoid!
But I've not been involved in the switch to newer batteries so I will leave that for others who have been there! I think there may be other points to consider but not my area to say???