Truck battery isolater built into 05 Arctic Fox camper?

Standard on Lance.

Well nwjetboat as an actual owner verified but curious why p/o labled I looked at the manual which seems pretty clear, its simply a disconnect turning off campers access to battery(s). Why the p/o labeled as described is curious, in the off position, which I think you verified should be no 12v in camper. In the on position allows camper to use battery (and in your case truck battery). You need to add solinoid or other wise isolate truck starting batt. And use some alchol to remove label.

http://www.northwoodmfg.com/documentation/CAMPEROWNER%202004.pdf
"You new truck camper is equipped with a 12-VOLT KILL SWITCH. The switch is located under the step to the bunk. The kill switch stops 12-volt power from the battery. To kill 12-volt power turn the
key until you can remove key. You may remove the key for security purposes, but it is not necessary to remove key to kill 12-volt power. Note that when the key is turned off or removed 12-volt power
will not be drained from the batteries–you need to turn the key on to operate with 12-volt power from the battery source.

The 12-volt kill switch should be used to avoid 12-volt drain if the unit is stored even for a short time period."

I have an 05 Arctic Fox, and as others have stated, the battery disconnect switch is for shutting 12v off TO the camper so there is no drain when stored. There is no isolator from truck to camper. I was going to install an isolator, but have made it a habit to just unplug as part of the camp set up routine.

Be careful what you wish for. For my old T C set up I added a relay to stop power to the TC when ever the eng was off. I soon found that this was a disadvantage because I would rather use both the truck and TC battery at the same time for the first evening, then isolate the TC the next day. This way I cut down on the depletion of the TC battery during the first evening and only minimally discharged the trucks battery.

Lance is about the only camper that I know of that has an isolator built into the campers and I don't know if it is an options or a standard build. AF does not have one. The battery disconnect shuts off the 12VDC to the camper for long term storage. It depends on your truck make if it needs an isolator or not. Ford shuts off the power to the charge line when you shut the key off. Chevy and Dodge do not. So if you leave the TC plugged in it can run down your batteries. If you have a GM or Dodge you can just unplug the line from the truck to the camper when you are going to be using the camper for a period of time without running the truck.

It is better to use a constant use solenoid under the hood to shut off power when you shut your truck down.

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.

If you carefully remove the positive lead on your TC battery you should have no votage on that lead. Becareful when you remove that lead. Make sure it doesn't touch any other metal parts. Now start your PU. Now you should have voltage on that lead. If there is voltage on that lead when your PU is not running then they are not disconnecting & you could run your PU battery down when parked. Personally I would prefer a solenoid over a manual switch because I am old & forgetful. Those solenoids only cost about $20.