Aha...part 1 of the mystery is solved! We had a bunch of rain late last night and early this morning. On a hunch, I went to look at the truck and sure enough, there was a very small puddle on the floorboard where the cab mounting bolt recess is. There is a leak in the cowl/windshield trim area letting water in. It was hitting the angled part of the floorboard and must have been going under the carpet, getting it wet, and around here it's too humid for anything to dry quickly, so it rusted, just as you'd expect.
I was wondering why the rest of the floor is so solid, but this makes total sense. I plan to keep it in the barn soon, but I'm still going to try tracking down the leak and sealing it. The nice thing is that the POR-15 creates something like an epoxy coating that is seemingly impervious to water...so it should be okay until I get it sorted out.