We have a electric/hydraulic grain trailer similar to yours, we also ran cable to truck battery, also have one setup on tractor too if we want to use it to pull it.  We used the ball as ground and ac/dc welder lead connectors to quick disconnect.  Cheap and easily replaceable from just about anywhere.  We have it and the trailer connections in the bumper of our trucks and use a piece of small angle iron to make a bracket to hose clamp it in place then chopped off the front face of a 6 pole trailer connector to act as a dust cover.   Works well and you never have to worry about the batteries being charged on the trailer.  Also keeps all the neighbors from borrowing it too! ;-)