6.2L Coolant Temperature

@MTF, the routing of the hose from the radiator cap to the reservoir hasn't been an issue for 78,000 miles, but sometimes the way you find the answer is to eliminate the non-answers. Rerouting the hose to a direct and level path around the airbox necessitated shortening its length by 4 1/2 inches. I doubt that this is what suddenly caused a problem and it doesn't address where the missing coolant is disappearing to, but I am happy to so easily eliminate a possible issue. (And I can't help but wonder why the factory unnecessarily wasted 4 1/2 inches of hose per F150 coming off the line.)

The coolant level in my radiator is and has been the same as yours - coolant right up the neck of the radiator to the cap. Given how the overflow reservoir works, it has to be.

Another thing I would suggest is to try and see how the two water heater hose connections look at the firewall.
You could have a small leak back there and you may not see it if it's dripping behind the padding.
Maybe stuff some paper towel around the bottom of each hose see if they get wet after driving a few miles.
It's a long shot but at least you can eliminate that area too.

Have gone down and dirty to visually inspect both plastic side tanks on the radiator?

All carpeting in the front footwells is dry. No sign of leakage there or at the radiator.

Dogonne it, my coolant is going SOMEWHERE!

UPDATE:

I refilled the reservoir to the bottom cold fill line, and it hasn't moved. I've driven into town twice since I got back from my trip, and the coolant level in the reservoir has remained unchanged.

That's good, but it doesn't explain where the missing coolant went. Puzzling puzzles like this puzzle me.