Flipping the trailer on-off battery switch is a personal decision, I suppose. The argument about the break-a-way cable needing the trailer battery to operate is true. But my break-a-way is wired into the battery cable, not through the camper. In other words, its attached to the battery before the trailer kill switch, so it's always active. It has it's own in-line fuse too.
So, if your break-a-way switch is independently fused before the trailer cut-off switch, I don't think it really matters if battery is on or off.
Charging the battery from the truck? Here again, trace our wiring. The battery may charge from the truck before the trailer cut off switch, not after it. Each trailer and each truck is set up different.