Mystery switch in battery locker roof.

Possibly  " How Old your van is " although you do say in your OP its a 2017 jobbie.

not everybody reads everything - I'm as guilty as anybody 

The van is a special from Ropers of Catterick. They are usually helpful so I would give them a ring.

Any update to this mystery?

I can see the wiring fom the mystery switch comes into the area underneath the front seat near the main loom but it disappears behind some panelling 

It looks as though it is a tracker from that description, although why you were not told about it when you bought the caravan is odd as it would be a selling point.

Maybe the original owner didn't tell the dealer they traded it in with and I bought it from.

phantom trackers are not transferable between van owners also the previous owner may not have renewed the subs for the tracker it cost about £99.for an annual sub but cheaper if you renew for 2 or 5 yrs 

you could check with Phantom to see if they have any record of any unit being fitted to the van The tracker unit is about the size of a box of matches and should site some where out of sight

Just a thought could it be the switched link on/off to the motormover control unit inside the caravan.

The main power switch (red) battery  to motormover is below.

Certainly if the battery is left connected then the mover control unit can be isolated when in storage saving power.

As mentioned it may be there for the same reason on a tracker.

It's not the motor mover as that works regardless of whether the switch is on or off.

I agree wirh the other reply it'll probably be the tracker 

Why not ask the dealer to contact the previous owner and ask them what it's for and then give you a call.