So I brought this home last night

Thanks for your persistence! lol
I confirmed with the dealer that originally sold the truck via the VIN it was sold with 20s. Not sure why it would have an 18" spare!

 

I think they all do, as long as the tire size of the spare matches the size of the tires that came on it when it was new it doesn't matter about the wheel size. Not many vehicles come with a matching spare wheel, especially trucks, remember the old little donuts on black wheels cars came with, some cars now don't even come with a spare. Now that you have different size tires your spare tire will not match in size but if you ever need it you should be fine using it for a short distance. Now if you were running a 35" or something really big then your spare would be worthless. The 3rd Gen Platinum's have only one wheel offering and is the 20" in the picture I posted