Don't beat yourself up about feeling those 18 wheelers when they blow by you. The simple physics is that an 18 wheeler passing you at 75 or 80 (yes, I live in Arizona), is pushing a huge bow wave of wind. There is nothing you can or should do to your rig to change that. About all you can and should do is pay more attention to that left mirror and simply anticipate a bit of a blow as they pass. I don't always see them coming (but I should), and even when they surprise me it is just a puff, nothing that is going to drive me to the shoulder.
I have a 2011 E-350 chassis and it rides and drives like a dream. Seems like most of the handling problems are found on the E-450's on the larger/longer C's. I have no plans to make any changes but we have lots of unused payload available. I have 3368# of payload and loaded for a typical trip are 1500# under that. :C