Battery Disconnect Switch

It probably is a battery disconnect switch for the whole trailer; no 12V power if it's turned off.

It would seem that the emergency breakaway brakes may be wired through this switch based on the warning label. If it's easy to modify that connection, it would be a good idea to do so in case you happen to be forgetful before towing.

Generally you'd have it on if you're towing or camping, and off for long-term storage where there's no shore power available, assuming it is a master disconnect switch

And long term storage would mean anything longer than a week. By the 2nd week your batteries will be on the low side from parasitic draws in the trailer.

Ah... I see said the blind man. My Montana is never off of shore power and has solar panels as well. I don't ever disconnect/remove the batteries or turn the switch off.

Mistake or not?

If your battery array is constantly being charged to 100% via solar everyday, should be no need to enable the battery disconnect. I would still verify, at least once, the batteries are being charged for confidence moving forward.