FWIW there's a good chance you can find in your owners manual that when saddlebags are fitted, speed should be limited to 80mph.
It's in my G/S manual and on a decal inside the lids of the old Denfelds that I saved when my RS moved on.
That is still a max recommend speed on some other bikes with bags installed. Also sounds like he was experiencing some of the handling gremlins around semis, not that uncommon in my mind. When you go away from a type of tire recommended by the manufacturer then you are on your own. It might handle fine then again it might not.