The bolts is all technical ************, if you overload the hitch it doesn't matter what grade bolts you have....
First of all you shouldn't be pulling 5k on a KJ unless your really set up for it, heavy duty cooling system, and even some aftermarket trans cooling to be safe. Not to mention your at the load rating of your hitch as well!!
I have seen 5k Class III hitches fail before the bolts holding the thing to the frame. Usually the welds holding the receiver tube, yes it does happen, i have seen that twice in my life, the first time was a guy that overloaded a trailer and had about 8k on his Class III, and the other was when we pulled out a truck with a 4wd tractor on the farm, pulled it right off. The rest of the hitch including the bolts were intact.
If your only pulling like 2-3k on average i wouldn't loose any sleep over the bolts that came with the hitch. Think of it this way, has there been any major recalls on them by any of the manufacturers, has there been any widespread reports of bolt failures on these hitches sold by eTrailer and elsewhere??
I read in a brochure once that Curt and Reese did testing to rate their hitches and this involved bolting the receiver to a fixed structure using the included hardware and loading them with dead weight until they fail. Once they know the dead weight rating they calculate a safe working weight rating. Its a lot different than dead weight in a towing situation, the trailer hopping like tjkj mentioned is a killer. I have broken the ball hitch on a utility trailer going over washboard roads with a load, usually the bolt that holds the piece that clamps against the ball fails in them, its not uncommon, i know a local boat dealer that says he repairs or replaces three or four per season for customers who overloaded or overstressed them on a bumpy gravel road. Ever wonder why safety chains are there? Take it from someone that experienced it first hand, thank goodness they are there! I had to hold the hitch onto the ball with a couple ratchet straps to get home.... Not fun!