If it was a good practice to put a switch in the negative side, OEM RV manufacturers and boat builders would also put the fuse and breaker panels in the neg side. They do not. In 15 years of marine work, I've never seen anything in the negative side of any power system.
The argument about the breakaway switch not working is a valid one. Also, all appliances and devices, both 12VDC and 115VAC share a common ground. I said ground, not neutral. That would make it possible to pick up a ground from anywhere in the trailer. While you would not get a large current surge as the neg side would be disconnected, you would still have a difference of voltage potential on everything in the trailer that is fed by the 12V. That is all that is needed to draw a small arc. That arc could be strong enough to damage electrical devices, particularly those controlled by solid state devices. By breaking the hot side, you are assured that everything in the trailer is dead. Like I said, if breaking the ground was a good practice, the OEM builders would do it.