Battery disconnect wiring

I tend to stay with existing convention where all switches and fuses are on the positive side.
Possibly an un-insulated knife switch I would put on the negative.

Break the positive as close to the battery as you can. This keeps the positive from grounding out further down the line and causing a short to ground.

Electrically it doesn't matter; an open in the circuit is an open in the circuit. Sometimes one or the other is easier to do (say because there are multiple positives going to the battery but only one negative).

For a trailer, I would take care to ensure that the breakaway braking system is not disconnected by any battery disconnect switch. It does not use any power unless activated. That very likely means putting the disconnect in the positive circuit, with the connection for the breakaway brakes on the battery side of the disconnect. There may be a few other loads that you also want to not be switched--such as the power for the disconnect solenoid, if you use a latching solenoid switch for that purpose, or a solar controller.

I would break the positive.