Below is a clip from a post I just made on a similar question about a disconnect switch.
My trailer came with a power disconnect switch on the positive terminal that serves me well. It doesn’t disconnect my slide out motors, landing gear nor rear stabilizer but it does turn off all my parasitic draws. If I were to add a battery cutoff switch, I would put it on the negative side of the battery for the same reasons you should disconnect the negative battery post first when removing a battery.
A wrench or tool accidently touching the negative terminal to ground on an installed battery has no effect. After first disconnecting the negative terminal of the battery, shorting the positive terminal to grounded trailer parts with your wrench also has no effect either because the negative terminal is open. On the other hand, accidentally shorting the tool to ground, while disconnecting the positive battery terminal first, can cause a large spark.
This may not be a major concern but it is the reason many advocate putting the battery disconnect switch on the negative terminal instead of the positive one. In either case, the battery is electrically disconnected from the trailer.
2015 Wildcat 282 RKX
2011 F350 Lariat, 6.7 L. Diesel, 8 ft., SRW, Reese R20