Wow...that's quite a history.
The Transit uses an oil-to-water cooler that exchanges heat between the transmission and the cooling system. Coolant contamination will destroy a transmission in short order, so that part makes sense. You can see contamination easily on the transmission dipstick, but of course Ford made this challenging to check by having the dipstick underneath the van.
Have they pressure tested the cooling system the latest time around?
His: 2015 F-150 XLT Crew, 3.5L EcoBoost
Hers: 2015 T-350 LWB LR wagon, 3.7L V6