Thank you, Fred, for bringing this to new owners' attention again. I might add that though the situation is unusual where one would have a heavy load already engaged when plugging in or starting the generator, it can happen.
Arcing can occur for example if, when breaking camp, a portable heater had been left on. I don't know that it would arc so much upon disconnect, but it likely would when you hook up or start the genset next time. Perhaps a coffeemaker was on at last disconnect; but some appliances may have circuits that don't auto restart motors or elements after a power loss - a switch must be deliberately reset. I wouldn't worry much about a regular RV fridge that takes a while to auto-switch from gas to electric, but if you've converted to it, a residential one might pull some juice and cause arc burns upon a subsequent power-up.
One concern that comes to mind is a hydronic electric element switch, left on at the last parking spot, that might draw some juice and cause sparking at the transfer contacts upon again plugging in or powering the genset. If the coach sat long after coming off the road, so that engine heat recycling had dissipated significantly from the hydronic unit, and the diesel burner was off, the element could conceivably move some juice across the contacts as they meet again. That little switch can easily be forgotten when breaking camp.
It's just a good idea to make sure such things are off when breaking camp. But where even the experienced among us can slip is when there's a sudden power outage in a park, and in our anxiousness to check it out we overlook shutting off the heater, hydronic element, residential fridge, or whatever might have been on at the time.
Joel