Just an update to this now that I have 2000 miles on my 2018 F150 HDPP.
I scaled it today now that I have added the cap and various accessories, and it weighed 5950 lbs. With the GVWR of 7850, I still have 1900 lbs of payload left. Rear axle weight was 2800 lbs so 2000 left there on its 4800 rating.
Haven't towed yet, but I did put 1300 lbs of UPS batteries in it for a recycle run the other week. I could tell I had some weight back there but it rode fine, handled fine, and was not even close to stressed out. As I would expect given I still had almost 1000 lbs of payload left (this was before the cap which I'd say added 250 lbs).
I will tow next week so we'll see how it handles that.
Mike