IMHO I also believe the addition of suspension upgrades, listed by Scarecrow; which I also had done on my 2014 a couple of months after purchasing my rig, increases the load carrying capacity of the rig. The only thing not addressed by these upgrades is "breaking power", so over-loading does reduces the ability to safely stop the vehicle in a potential crash event. Knowing this I drive "55 to Stay Alive" when ever on the interstate and freeways, and always in the right lane whenever possible.
Nothing you have done will increase the capacity rating of the vehicle. Check out the fine print on websites for all your "improvements".
The things that stop the vehicle are
Brakes, not
Breaks. (I am a sometimes believer in the if you cannot spell it, perhaps you should not make statements on the subject)
Any increase in weight of the extra stuff you had installed comes out of the CCC.
As others have stated, is is possible to decrease the capacity by installation of inferior tires.
There are freeways in America where driving 55 puts yourself and others at risk by jamming up the flow of traffic, and causing more opportunities for crashes in the otherwise unneeded congestion. there is a place for keeping up with the flow of traffic. Best schedule your travel in these places for off-peak periods if you insist on driving overloaded and want to crawl down the highway.