May be a difficult one to answer but here goes, My 2012 350 diesel just turned 80,000 km. Out of this I think about 40k was towing a 8k fiver for three years and this year a 15 k toyhauler. Thinking ...
You should be doing an annual brake inspection, then you would know the extent of wear. In PA, it is required. There is no need to replace brakes based on miles, every truck is driven differently on different roads.
Road salt can really cause problems, especially on late model Ford trucks, so check for wear, rust and caliper sticking.