Probably a good idea to have the breaker off when hooking up too. I find at most campgrounds the breakers are off when we arrive, and have seen campground workers turn them off when they are cleaning the sites after the sites are vacated if they were left on. I always have the breaker off when connecting and disconnecting.
The male cord cap on the trailer's service cord and the pedestal receptacle are considered a disconnecting means .
That being said you should not plug in or unplug an appliance or trailer when under load. The circuit breaker should be in the off position , then the trailer cord plugged into the pedestal receptacle and then the circuit breaker turned to the on position.
Plugging / unplugging the trailer under load damages the male cord cap and the pedestal receptacle due to arcing.