I guess I'm not making myself clear
No, it seems pretty clear that you don't want to check RockAuto or the other sites I suggested for the tank.
Backordered from Ford. None in any warehouse in the country.
Great. What about all the Ford dealerships
in the country? Many of them (including the one I worked at) have old parts sitting on their shelves - not in warehouses; in the dealerships. The ones OUTSIDE the Phoenix area.
And I promise you that the system available to Ford dealership parts guys does NOT check "every warehouse in the country". At best, it checks every FORD warehouse. It doesn't check RockAuto's warehouses, or any other independent parts dealers'. So YOU have to check them.
THAT's the unclear part. If you don't verbosely list every place you've checked, how will we know which ones you didn't, to suggest them to you?
But you're making a tempest in a teacup. You have no reason to believe your tank will fail in the foreseeable future. And even if you change it, the new one is exactly as likely to fail (if not moreso). I'd be more concerned with tires (including the spare), ATF, lights, wipers, brakes... Things that are actually known to wear out frequently, and that you'll depend on to complete the trip. You can't plan for everything, so don't try. Plan for the likely things, which are conveniently listed for you in the maintenance schedule.
The coolant reservoir isn't on it, for a reason.