OK....here is my take on it. You have to do some research and I did it quickly....so PLEASE take this as a SUGGESTION....and do your own due Diligence.
First, from the following link.
Scroll down to page 19. These are the values for a SINGLE Shear load. Note that the value is about 13,600 Lbs for a typical, 5/8" Draw Bar pin.
NOW, go to the following link.
If the link breaks, then google Shear Strength 5/8" grade 5 bolt.
Scroll down and there is a good explanation of Single versus Double Shear. SO, in the above, we used Single....so lets use SINGLE to keep it Apples to Apples. The Single Shear for a Grade 5 bolt is 22,100 Lbs. If you look to the left, a standard Grade 2 bolt is 13,600. HMMM....we've seen that before.
Therefore, a piece of 5/8 bar stock (probably 1038) whether used in a Bolt or as a Drawbar pin is the same. NOW, if you upgrade the metallurgical properties of the bolt to grade 5, it goes up almost 90%. That is because the core hardness of the bolt is harder. Obviously it has to be properly case hardened and also stress relieved to achieve the correct values.
Bottom line, a Grade 5 bolt, properly torqued will obviously be stronger. NOW, comes the practical side of it. On MY Roadmaster XL, I have TWO 5/8" pins connection the baseplate arms to the hitch receiver. They are the equivalent of a Grade 2...but there are two of them...so you have, in theory, 13,600 Lbs per side. However, in actual use, the load varies from near zero to near 100%.
NOW (hang in here with me....there IS a conclusion and a recommendation), you have TWO Grade 2 pins holding the hitch spreader bar to the tow bar. They are 3/4", or 19,600 Lbs. The Hitch has two pivot bolts, which are (probably) Grade 5 or 31,800 Lbs. Therefore, if you have the hitch in a position where ONE side (one Baseplate) is carrying the load, you would have ONLY 13,600 Lbs or the rated load of a SINGLE drawbar (5/8") pin.
Therefore the weakest link in the whole assembly is the drawbar pin in the MH Receiver and the drawbar pin in the baseplate arm. They are EQUAL....
Therefore, if you increase the shear strength of ONE, you have to kick up the shear strength of the OTHER. In my case, I would have to use a Grade 5 bolt on BOTH of the arms and also on the MH Receiver.
Assuming THAT I did that....you would need to properly torque the bolt(s). You should use a FLAT washer, not a SPLIT one. You should use a Grade 5 nut and torque it to 128 Ft. Lbs. See the link below. This assumes you use zinc plated fasteners and NO loctite. The K factor is 0.17.
128 Ft. Lbs. is a LOT of Torque. Once you torqued the hitch bolt with a nut, you really SHOULD add a "vibration resistance" nut. So you use a Nylock Nut to "Lock" the first nut in place. Now you have to have both an open end wrench as well as your socketed torque wrench. You PROBABLY have room to do that on the MH.
BUT, I would NOT, I think have any WAY of getting the torque (to the proper MEASURED value) on the XL Baseplate arms. There is NO room for a torque wrench (with a socket). YES, you could use a Crow's foot attachment, but you would have to use a higher value.
Therefore, unless you intend to replace the Drawbar pins in a typical Roadmaster mount, why do it? I pull my tow bar off (it is locking) and do not leave the MH. It would be quite a task to bolt and unbolt it each time.
NOW, if you wanted an additional 25% increase in shear, some of the Tractor manufacturers have a "high grade" 5/8" Hitch Pin. New Holland has one that is PN SPC71021WCDS. It is much longer than your standard Draw Bar as the usable length is 4 1/4" versus 3 3/4". It also has a handle, so it will take MORE clearance. IF you used that for the MH Drawbar, you SHOULD drill out the hitch pin clip hole. You need to drill them to 5/16" to accomodate most locks. That is what I did with mine as I lock ALL the hitchpins.
I doubt, based on clearance, that I can get the longer ones, with the handles, in where the bent ones go to attach the arms to the baseplates.
BOTTOM LINE....you probably have ENOUGH safety factor in the 5/8" pins. Swapping the MH pin for a hardened bolt and then properly torquing it and adding a properly torqued Nylock nut gives you MORE shear strength in attaching the Tow Bar to the MH. BUT, the load on each individual draw bar pin (assuming you have the Roadmaster XL series) can approach 100% and you gain nothing. However, it will take a LOT of effort to remove or sabotage your MH attachment.
BTW, if you look at the classes of 2" receiver hitches, the max is 14,000 Lbs for a Class IV. If you upgrade to a Class V, it is a 2 1/2" receiver. BUT, all the 5/8" hitch pins are rated for Class I to V. Therefore, unless you find a specially hardened or higher strength draw bar pin and also BEEF UP the Base Plate arm attaching system, you gain nothing.....
Blue Ox.....Have NO idea how they rate the attaching arms or whatever.
There WILL be a pop quiz....so read this again....