I worked with a guy had a 2016 Dodge Cummins 4WD. It had rust over the back wheels barely a year after he got it, and he kept it spotless. Dodge does seem to be the worst, but I have seen other brands of newer truck with the bed rot.
Yeah the D Series trucks are known for the drip rail rot. Seems to be no rhyme or reason...I've seen rotteed to death trucks with good cab roofs and clean bodies with rusty roofs.