The isolation relay/solenoid would have to be on the truck and would not be on the TC. Power from truck to TC is with the ignition key in the 'On' position, and your TC doesn't know when the truck is on or not.
Dodge supplies power to the 7-way trailer harness regardless of ignition key position. So if there isn't an aftermarket device on the truck, it will be supplying power if you do not disconnect the TC.
The battery disconnect box under the sink is most likely a simple disconnect switch that will disconnect the TC from the TC batteries. The truck and TC batteries will remain connected together regardless of disconnect position. But since the previous owner has placed labels on this disconnect, he may have altered the wiring so that the disconnect allows him to cut the truck supply power to the TC batteries, and you have lost the ability to kill all power to the TC 12V system.
Reading my above may get a little confusing...
Sounds like the previous owner has altered the disconnect switch to disconnect the truck from the TC. If you can remember to flip the disconnect on/off, you can leave it as is. If you want it to be automatic, you will need to add an isolation relay to the truck wiring.
