From what you posted it was not a warranty issue......that's obvious.
If you had a new central air and heat installed in your house and then had drywall and flooring replaced right near the return air vents(while the unit was running the whole time) then the unit iced up because of no air flow, would you think that should be covered under warranty even though dust from the flooring/drywall work plugged up the evaporator? The A/C guys would tell you to go you know where.
Your fault accept it! Removing the plastic casing around the evap coil takes about 2 hours to remove and clean, no big deal.
Extremely dusty environment every day and the evap is plugged up, you don't know if the other truck owners were running the A/C or had the vents on mix or anything......maybe they even have a cabin air filter.
Ford didn't cover the repair under warranty and I wouldn't have either. Extremely dusty conditions are over and above the control of Ford Motor Company because you should have had a cabin air filter installed if you work in said conditions.
I have worked in the A/C and refrigeration field for 10 years and have heard every excuse....yours is a popular one.
I think a cabin air filter should be standard on every vehicle with A/C.......after all if you ride around as much as a lot of people on this forum you are breathing air passing though that nasty dirty coil.
If you think about it........can you really blame Ford?