I like the idea of having the chains on the vehicle and being criss-crossed with the break away lanyard being short enough that it activates the brakes before the chains fully extend.
My original rational for asking the question of frame vs receiver was because I saw the receiver as being less reliable than the vehicle frame. I did not really consider the tongue dropping of the ball, which is a separate point.
Just to throw some fuel on this....wouldn't a weight distribution system assist in keeping the tongue on securely?