Well.. the NCV3 is much more complicated than the T1N (in the T1N, the only thing that really cared that the battery was disconnected was the radio... you had to re-enter the security code)
First, the mandatory lesson: starter batteries don't like a steady low load that drags them down. They're engineered for short high-current usage.
Using them in a situation where a deep cycle "marine"-type battery would be better shortens their service life.
So, if you could add an auxiliary ("house") battery charged by an isolator from the alternator, you'd be better served.
That said, where and how are the "accessory" loads attached?
Are you speaking of things plugged into the 12v power sockets, or stuff like the built-in cabin lights?
And (since i don't have an NCV3) what parts of the Sprinter do fall into a funk if you disconnect the battery?
Looking at a 3rd party circuit diagram, i'd guess that power distribution block (PDB) fuse number 3 definitely wants to remain alive.
Three of the four 12v DC outlets do not use a fuse in the PDB, its bus bar connects to a thickish red wire that feeds the seat-side fuse blocks, where fuses 23, 24 and 25 feed the sockets (if my diagram matches your fuse assignments).
That bus bar connection also feeds the windshield heater's 30amp fuse, and the air conditioner's 50 amp fuse. If you can locate them, it may help back-track the wires to the bus bar connection on the PDB. Putting your disconnect to open the "red wire" circuit there would probably be "safest". (note: all of the powered connections on the PDB are red wires, so the color alone doesn't really help).
(the other unfused connection on the PDB goes to the starter motor and jump-start post)
If it's the interior lights you're trying to kill... well, that's much harder... they're probably controlled by the "body control module", (BCM) which handles a surprising number of subsystems. Fuse 5 on the PDB will kill many subsystems which appear to be non-critical (or not trying to remember things) associated with the BCM. It will also kill the radio.
good luck
--dick