Don't take offense, but tag hitch is just all wrong from an injunearing standpoint.   Be one heck of a lot stronger if the vertical member were a piece of say 1/2x5 bolted on the side of the frame instead of just being hung on the bottom flange.       Should't be bolting to the flange at all, and definitely not with so much prying load on it.  

Don't mean to be critical, but also wouldn't want you to have to fish something out of the ditch.