My own TM has something much more complicated and expensive (which you shouldn't try to duplicate).
A 5-pin "Bosch style" 12v relay can be bought at any car parts store, or Amazon, or eBay) but you also need a socket (converting the Relay "pins" into easily connected wires. Her'es an Amazon listing for both items:
https://www.amazon.com/Augeny-Car-Re.../dp/B0CNLBW59N
The Relay can go behind the fridge. After breaking the 3-way junction of the TBC wire, the wire which goes down, out and forwards to the breakaway switch should be connected to relay pin 30 (power "common", the fat wire with light orange insulation in the harness from Amazon).
The thin TBC lead from the bargman cable needs a new "splitter" junction. One downstream wire connects to relay pin 87. That pin is connected with pin 30 when battery power fails, and the relay coil magnet loses power. The other "downstream wire" is input 12V to the DC->DC charger.
Because the back of the refrigerator gets very hot, I would not put the DC->DC battery charger in that location. So that second "downstream wire", which is new, goes somewhere else to reach the DC->DC charger unit. (maybe under the sink, or near the WFCO, or maybe alt the way back to nearly reach the batteries). Near the batteries is best. This wire does not connect the WFCO 12v fuse board anymore, it connects only as "input" to the the DC->DC boosting charger unit.
The "+12V output" of the DC-to-DC boosting charger should go a short distance, reaching a 12V battery connection. (it's usually operating at more than 12v, of course.)
This leaves one unused and loose wire end behind the fridge. (It used to go from the WFCO 12v fuse board into the old TBC junction.) it should now be connected into another "spliiter" junction, connecting both pin 87A (the big blue wire on Amazon's socket) and pin 85 (the thin yellow-insulated "12v" wire for the coil.
The thin white wire of the Relay base (the coil "ground") be be interconnected to any other 12v grounding wire or grounding lug behind the fridge.